CODEOWNERS: stm32: Update on active members

In early Zephyr days, STM32 code base greatly benefited from the care
of a handful of people.
These days are gone and so did these few people that haven't been
contributing for at least a year now.
CODEOWNERS file is updated to reflect current status of contribution,
for the areas where the is an active contributor.

Let them be thank for these initial contributions.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>