Luben has not been active and has not responded to mails since summer
2024. Remove him from MAINTAINERS and add an entry in CREDITS.

Philipp has a new email address and an ACK to commit work time to the
scheduler. Thus, set the state to 'Supported'.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113121851.31382-1-phasta@kernel.org