MAINTAINERS: Update my email address to @kernel.org

MAINTAINERS: Update my email address to @kernel.org

I had issues using my gmail account multiple times this week, hence
switch to @kernel.org.

Signed-off-by: Boqun Feng <boqun@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Gary Guo <gary@garyguo.net>
Acked-by: Kunwu Chan <kunwu.chan@linux.dev>
Link: https://patch.msgid.link/20260129004520.62697-1-boqun@kernel.org
[ Sorted alphabetically in mailmap. Removed Cc tag list. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>