Deprecate ca_signature_algo config

[marcoandredinis]

After the merge of #12674 we no longer use the following configuration:

teleport:
    ca_signature_algo: "rsa-sha2-512"

As we now rely upon the x/crypto package to choose the signing algorithm (it defaults to rsa-sha2-512)

Demo
If we set ca_signature_algo (the value is irrelevant) and start teleport we get:

root@marco:/workspace# teleport start --debug
2022-06-02T09:33:58Z WARN             ca_signing_algo config option is deprecated and will be ignored, we'll always default to rsa-sha2-512. config/configuration.go:348
2022-06-02T09:33:58Z INFO             Generating new host UUID: b001159a-10e0-49a7-b4dc-61c73fbe9e42. service/service.go:726
...

Fixes #12905

[codingllama]

Thanks for the cleanup, @marcoandredinis !

[codingllama]

Let's add a deprecation notice at fileconf.go / Global.CASignatureAlgorithm too, so it's clear the field is unused and kept only for config backwards compatibility.

Is there a "Cloud"/protobuf counterpart for this setting, or just the fileconf / teleport.yaml one?

[marcoandredinis]

Deprecation notice added 🙏

Is there a "Cloud"/protobuf counterpart for this setting, or just the fileconf / teleport.yaml one?

CertAuthorityV2 has two methods that I'm pretty sure could be removed: Get/SetSigningAlg

My concern is having a version without those methods interacting with a version that has them
I don't know how that works out (backwards/forward compat)
I'll do some tests 🛠️

[codingllama]

If you drop the methods and an older gRPC client tries to call them, it'll get a framework-level error saying the method doesn't exist (I don't remember the exact error, but it's something like that).

Whether it's safe to drop depends on how it's used. If you can ensure all calls are removed by version N, then it's safe to drop the method in N+1.

[codingllama]

LGTM Marco, let me know if you intend to do further work in this PR, otherwise I'm happy to approve.

[zmb3]

Looks good, but there's also a reference to ca_signature_algo in our docs - please update that as well.

[marcoandredinis]

@zmb3 @codingllama
Can you please take another look?
I removed some more files, and I'm confident the only leftovers are the gRPC messages, which can be removed in the next major version

I tested using teleport built from this branch against tsh v9.3.2 and it worked:

root@moon:/workspace/teleport# tsh version
Teleport v10.0.0-dev git:teleport-connect-preview-1.0.0.dev.1-252-g855e461ca go1.17.10
Proxy version: 10.0.0-dev
root@moon:/workspace/teleport# tsh ssh root@marco.mydemo
root@marco:~# exit
logout
the connection was closed on the remote side on  03 Jun 22 10:45 UTC
root@moon:/workspace/teleport# ./tsh version
Teleport v9.3.2 git:v9.3.2-0-gb3597d12a go1.17.10
root@moon:/workspace/teleport# ./tsh ssh root@marco.mydemo
root@marco:~# exit
logout
the connection was closed on the remote side on  03 Jun 22 10:45 UTC

[codingllama]

LGTM.

All suggestions are about removing redundant initializations. Feel free to push back/ignore as you see fit.