docs: change security advisories notification channel

docs: change security advisories notification channel

Our documentation currently says that new security advisories will be
announced on the project's issue tracker. However, this issue tracker
is barely used by TF-A community and the software it is based on is
getting deprecated. Thus from now on, security advisories will rather
be announced on the project's mailing list.

Update TF-A documentation to reflect that.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: If2f635795e0af4c794015a025899bfcc7116ab38