gdb: use dynamic year in update-freebsd.sh

gdb: use dynamic year in update-freebsd.sh

When running update-freebsd.sh on FreeBSD, I see the following
modification in freebsd.xml,

-<!-- Copyright (C) 2009-2023 Free Software Foundation, Inc.
+<!-- Copyright (C) 2009-2020 Free Software Foundation, Inc.

It means that each time, when we running the update-freebsd.sh on
FreeBSD, we have to correct the year of copyright manually. So fix this
issue by using dynamic year.

Tested by regenerating freebsd.xml on FreeBSD/amd64.

Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Simon Marchi <simon.marchi@efficios.com>