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 <[email protected]>
Change-Id: If2f635795e0af4c794015a025899bfcc7116ab38

Author: sandrine-bailleux-arm

docs/process/security.rst

@@ -9,10 +9,8 @@ relevant to Trusted Firmware-A. We encourage responsible disclosure of
 vulnerabilities and inform users as best we can about all possible issues.
 
 We disclose TF-A vulnerabilities as Security Advisories, all of which are listed
-at the bottom of this page. Any new ones will, additionally, be announced as
-issues in the project's `issue tracker`_ with the ``security-advisory`` tag. You
-can receive notification emails for these by watching the "Trusted Firmware-A"
-project at https://developer.trustedfirmware.org/.
+at the bottom of this page. Any new ones will, additionally, be announced on the
+TF-A project's `mailing list`_.
 
 Found a Security Issue?
 -----------------------
@@ -86,4 +84,4 @@ Security Advisories
 
 --------------
 
-*Copyright (c) 2019-2022, Arm Limited. All rights reserved.*
+*Copyright (c) 2019-2023, Arm Limited. All rights reserved.*