DEVELOPERS: Free Electrons is now Bootlin

Free Electrons is being renamed to Bootlin. While the
@free-electrons.com e-mail addresses still work, it is not guaranteed
to be the case in the future. Hence, this patch updates the DEVELOPERS
file to use the @bootlin.com addresses for all Bootlin engineers.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>