MAINTAINERS: update David Hildenbrand's email address

MAINTAINERS: update David Hildenbrand's email address

Switch to kernel.org email address as I will be leaving Red Hat.  The old
address will remain active until end of January 2026, so performing the
change now should make sure that most mails will reach me.

Link: https://lkml.kernel.org/r/20251103103659.379335-1-david@kernel.org
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>