Use the Go 1.21.0 toolchain to build modules

[codingllama]

This has (mostly) no practical effect as of right now, but it's a convenient way to bump the Go version for future patches/releases.

Done using go get toolchain@go1.21.

• https://go.dev/blog/toolchain

[codingllama]

Dropped "Build CI Buildbox Images" changes in favor of #31173.

[codingllama]

Also dropped api/go.mod toolchain line, as it won't build directly with Go 1.19 because of that.

We could keep the toolchain line and it should still be go-gettable, or at least that's what I'm led to believe, but then we'd have to change the "Build API Module" action to use Go 1.21+.

$ cd "$(mktemp -d)"
$ go1.19.12 mod init test
> go: creating new go.mod: module test

$ go1.19.12 get github.com/gravitational/teleport/api@3991c36
> go: added cloud.google.com/go/compute v1.20.1
> go: added cloud.google.com/go/compute/metadata v0.2.3
> go: added github.com/gravitational/teleport/api v0.0.0-20230829195104-3991c368e931

$ cat go.mod
> module test
> 
> go 1.19
> 
> require (
> 	cloud.google.com/go/compute v1.20.1 // indirect
> 	cloud.google.com/go/compute/metadata v0.2.3 // indirect
> 	github.com/gravitational/teleport/api v0.0.0-20230829195104-3991c368e931 // indirect
> )

[jakule]

CI is not happy

/__w/teleport/teleport/go.mod:5: unknown directive: toolchain

[codingllama]

CI was running a pre-Go 1.21 image, see #31173. I didn't manually re-run all the failed jobs, but judging from Unit Tests I think we are good now.

[jakule]

If CI gods are happy, I'm not going to disagree with them.

[codingllama]

This is obsolete since #31231.