ci: remove circleci workflow

[crazy-max]

follow-up #3100

- What I did

Our GitHub Actions workflows are up for quite some time and look to work fine so let's remove CircleCI 🔫

Also I see CircleCI complains again when checking out the repo: https://app.circleci.com/pipelines/github/moby/swarmkit/578/workflows/835dd771-7346-4845-87b3-2da849f152ad/jobs/10967?invite=true#step-101-81

Using SSH Config Dir '/home/circleci/.ssh'
git version 2.38.0
Cloning git repository
Cloning into '.'...
remote: Enumerating objects: 45535, done.
remote: Counting objects: 100% (8732/8732), done.
remote: Compressing objects: 100% (4553/4553), done.
remote: Total 45535 (delta 3876), reused 8135 (delta 3621), pack-reused 36803
Receiving objects: 100% (45535/45535), 39.05 MiB | 36.42 MiB/s, done.
Resolving deltas: 100% (27980/27980), done.
Checking out branch
fatal: reference is not a tree: 75e563b4b3c63a740ac0aa413ad33c039d891f63

exit status 128

When merged, maintainers having access to the repo settings should cleanup webhooks and GitHub Apps related to CircleCI. (cc @dperny @neersighted @corhere @thaJeztah)

- How I did it

Remove CircleCI workflow

- How to test it

- Description for the changelog

[crazy-max]

Some flaky tests: https://github.com/moby/swarmkit/actions/runs/4114820437/jobs/7102711665#step:5:75

time="2023-02-07T14:23:28Z" level=error msg="update failed" error="task z562qp2reo3s10tfjrrf7l1es was already shut down when reached by updater (state: SHUTDOWN)" task.id=3yfb47whxdxp3wivx0i6a912j
--- FAIL: TestUpdaterRollback (6.38s)
    --- FAIL: TestUpdaterRollback/pause/monitor_set/spec_version_set (3.60s)
        update_test.go:272: service was updated
        testutils.go:117: goroutine 128 [running]:
            github.com/moby/swarmkit/v2/manager/orchestrator/testutils.FatalStack(0xc0002cab60, {0x11f19d7, 0x10}, {0x0, 0x0, 0x0})
            	/go/src/github.com/docker/swarmkit/manager/orchestrator/testutils/testutils.go:116 +0x71
            github.com/moby/swarmkit/v2/manager/orchestrator/testutils.WatchTaskCreate(0x1349840?, 0xc0002e9740)
            	/go/src/github.com/docker/swarmkit/manager/orchestrator/testutils/testutils.go:43 +0x225
            github.com/moby/swarmkit/v2/manager/orchestrator/replicated.testUpdaterRollback(0xc0002cab60, 0x0, 0x1, 0x1)
            	/go/src/github.com/docker/swarmkit/manager/orchestrator/replicated/update_test.go:343 +0x22a5
            github.com/moby/swarmkit/v2/manager/orchestrator/replicated.TestUpdaterRollback.func2(0x0?)
            	/go/src/github.com/docker/swarmkit/manager/orchestrator/replicated/update_test.go:21 +0x38
            testing.tRunner(0xc0002cab60, 0x12430d0)
            	/usr/local/go/src/testing/testing.go:1439 +0x214
            created by testing.(*T).Run
            	/usr/local/go/src/testing/testing.go:1486 +0x725

Already tracked in #2559

---

And integration tests panics: https://github.com/moby/swarmkit/actions/runs/4114820437/jobs/7102712007#step:5:14553

time="2023-02-07T14:21:51Z" level=info msg="5fe84f4008cbcf69 switched to configuration voters=(6910860764644101993)" module=raft node.id=6d82hm2w2fny56uky3rlt6dlh testname=TestMixedFIPSClusterNonMandatoryFIPS testnode=4
time="2023-02-07T14:21:51Z" level=info msg="5fe84f4008cbcf69 switched to configuration voters=(4258104888426337958 6910860764644101993)" module=raft node.id=6d82hm2w2fny56uky3rlt6dlh testname=TestMixedFIPSClusterNonMandatoryFIPS testnode=4
time="2023-02-07T14:21:51Z" level=info msg="5fe84f4008cbcf69 switched to configuration voters=(4258104888426337958 5076178877497517513 6910860764644101993)" module=raft node.id=6d82hm2w2fny56uky3rlt6dlh testname=TestMixedFIPSClusterNonMandatoryFIPS testnode=4
time="2023-02-07T14:21:51Z" level=panic msg="tocommit(54) is out of range [lastIndex(45)]. Was the raft log corrupted, truncated, or lost?" module=raft node.id=6d82hm2w2fny56uky3rlt6dlh testname=TestMixedFIPSClusterNonMandatoryFIPS testnode=4
panic: (*logrus.Entry) 0xc00cd68850

goroutine 27071 [running]:
github.com/sirupsen/logrus.(*Entry).log(0xc00c87c230, 0x0, {0xc010750a80, 0x5d})
	/go/src/github.com/docker/swarmkit/vendor/github.com/sirupsen/logrus/entry.go:260 +0xa07
github.com/sirupsen/logrus.(*Entry).Log(0xc00c87c230, 0x0, {0xc00c0e6628, 0x1, 0x1})
	/go/src/github.com/docker/swarmkit/vendor/github.com/sirupsen/logrus/entry.go:304 +0x8c
github.com/sirupsen/logrus.(*Entry).Logf(0xc00c87c230, 0x0, {0x17a864d, 0x5d}, {0xc00c69e600, 0x2, 0x2})
	/go/src/github.com/docker/swarmkit/vendor/github.com/sirupsen/logrus/entry.go:349 +0xc5
github.com/sirupsen/logrus.(*Entry).Panicf(0xc00cb60000?, {0x17a864d, 0x5d}, {0xc00c69e600, 0x2, 0x2})
	/go/src/github.com/docker/swarmkit/vendor/github.com/sirupsen/logrus/entry.go:387 +0x65
go.etcd.io/etcd/raft/v3.(*raftLog).commitTo(0xc00cb60000, 0x36)
	/go/src/github.com/docker/swarmkit/vendor/go.etcd.io/etcd/raft/v3/log.go:237 +0x167
go.etcd.io/etcd/raft/v3.(*raft).handleHeartbeat(_, {0x8, 0x5fe84f4008cbcf69, 0x467235cb5994a5c9, 0x4, 0x0, 0x0, {0x0, 0x0, 0x0}, ...})
	/go/src/github.com/docker/swarmkit/vendor/go.etcd.io/etcd/raft/v3/raft.go:1508 +0x6a
go.etcd.io/etcd/raft/v3.stepFollower(_, {0x8, 0x5fe84f4008cbcf69, 0x467235cb5994a5c9, 0x4, 0x0, 0x0, {0x0, 0x0, 0x0}, ...})
	/go/src/github.com/docker/swarmkit/vendor/go.etcd.io/etcd/raft/v3/raft.go:1434 +0x625
go.etcd.io/etcd/raft/v3.(*raft).Step(_, {0x8, 0x5fe84f4008cbcf69, 0x467235cb5994a5c9, 0x4, 0x0, 0x0, {0x0, 0x0, 0x0}, ...})
	/go/src/github.com/docker/swarmkit/vendor/go.etcd.io/etcd/raft/v3/raft.go:975 +0x16d5
go.etcd.io/etcd/raft/v3.(*node).run(0xc010db78c0)
	/go/src/github.com/docker/swarmkit/vendor/go.etcd.io/etcd/raft/v3/node.go:356 +0xe85
created by go.etcd.io/etcd/raft/v3.RestartNode
	/go/src/github.com/docker/swarmkit/vendor/go.etcd.io/etcd/raft/v3/node.go:244 +0x2be
FAIL	github.com/moby/swarmkit/v2/integration	54.213s

[codecov-commenter]

Codecov Report

Merging #3114 (64c0f52) into master (8892b33) will increase coverage by 61.75%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #3114       +/-   ##
===========================================
+ Coverage        0   61.75%   +61.75%     
===========================================
  Files           0      153      +153     
  Lines           0    31099    +31099     
===========================================
+ Hits            0    19206    +19206     
- Misses          0    10358    +10358     
- Partials        0     1535     +1535     

[thaJeztah]

LGTM, thanks!