semver: deprecate semver.Max

There are a number of calls to semver.Max in the wild, so let's not
delete it yet.

For golang/go#32700

Change-Id: I515b05f174195d1b82a931b92e21b83c370ef7c7
Reviewed-on: https://go-review.googlesource.com/c/mod/+/269357
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>