go.mod: update required go version to go1.22

Now that go1.23 has been released, versions of Go older than go1.22 are
no longer supported.

This will allow us to use the go/version package, which was introduced
in Go 1.22.

This change will force modules that depend on golang.org/x/mod, notably
golang.org/x/tools, to update their Go version requirement to at least
go1.22 when they update their requirement on golang.org/x/mod to a
version after this commit.

For golang/go#63395

Change-Id: I6f6b5bb9e43b5f9945cc5bc8c398628436d2e739
Reviewed-on: https://go-review.googlesource.com/c/mod/+/605796
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

go.mod

@@ -1,5 +1,5 @@
 module golang.org/x/mod
 
-go 1.18
+go 1.22
 
 require golang.org/x/tools v0.13.0 // tagx:ignore