Access request compatibility for servers without v2 api

[xacrimon]

Fixes a compatability issue introduced by security fixes where the client would call a grpc method that doesn't exist.

[codingllama]

Could we add a test for this?

[r0mant]

lgtm once @codingllama's comments are addressed

[xacrimon]

@codingllama Please take another look now

[codingllama]

Hey @xacrimon, the main ask here was if we could add a test for this. WDYT?

[xacrimon]

@codingllama Ah, the emails I got from GitHub had that first comment collapsed. If what you're asking is if we can precisely test the compat to make sure it hits the fallback. I'm not sure on how to do that to be honest, you'd have to somehow mock gRPC into thinking a method is unimplemented (otherwise we'd end up with a lot of wacky interfaces and indirections everywhere), I'm not aware of any other similar tests in Teleport but if you have any ideas I'm all ears.

[codingllama]

Approving, as it seems unfair to hold you on something we don't seem to do. We can do a follow up if you like.

if you have any ideas I'm all ears.

I think creating a new gprc.Server and binding proto.UnimplementedAuthServiceServer to it would do it.

[github-actions]

@xacrimon See the table below for backport results.

Branch	Result
branch/v8	Create PR
branch/v9	Create PR