main.star: drop release-branch.go1.21

The Go 1.23.0 release is out, so 1.22 is the oldest supported release.¹
The previous infrastructure (cmd/coordinator) does this automatically,²
whereas here I authored this CL manually. We'll explore automating it
later on³, possibly with relui generating the CL.

This also prepares to drop the special cased extra branches for x/tools
per plan in go.dev/issue/65917, but does so in the follow up CL so that
it can be submitted slightly later, separately.

¹ https://go.dev/doc/devel/release#policy
² It too used to be manual, until we automated it in go.dev/issue/34097
  in 2019.
³ I said the same thing 6 months ago in CL 562395. That said, we need
  to do a bit more one-off changes here since x/tools is dropping its
  extended Go testing.

For golang/go#65917.

Change-Id: Id5228b484995ded1094aaeb6b747f2f507ed241d
Reviewed-on: https://go-review.googlesource.com/c/build/+/605295
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>