release: remove manual regression testing from release process

[bobheadxi]

Once the state of automated QA testing is stabilized in https://github.com/orgs/sourcegraph/projects/90, we can probably replace all the manual regression testing in the release process with:

• Tag release
• Check buildkite
• File release-blockers from buildkite output

As of 3.22, the manual testing steps are documented in https://github.com/sourcegraph/sourcegraph/issues/15023 . Note that deploy-sourcegraph-docker comes with its own set of manual testing as of 3.22: https://github.com/sourcegraph/deploy-sourcegraph-docker/blob/master/RELEASING.md