go/types, types2: use go.dev/issue/nnnnn when referring to an issue (cleanup)

go/types, types2: use go.dev/issue/nnnnn when referring to an issue (cleanup)

Apply the following regex substitutions, in order:

golang/go#(\d+)  =>  go.dev/issue/$1
issue #?(\d+)    =>  go.dev/issue/$1

Providing a link uniformly makes it easier to find the respective issue.

Change-Id: I9b60ffa1adb95be181f6711c2f171be3afe2b315
Reviewed-on: https://go-review.googlesource.com/c/go/+/462856
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>