Maintainers.txt: update Gary's email address

Maintainers.txt: update Gary's email address

I've left SUSE last month, so the original email address is not
functional anymore. Update my email address to the new one.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Gary Lin <gary.lin@hpe.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>