deprecate use of GnuPG ids for APT keys

URLs to keys should be used instead.

Change-Id: Ieceffb7b0ae941cad08fcea1a752ad021e7ad855

releasenotes/notes/deprecate-apt-keys-eff6e68272923b6b.yaml

@@ -0,0 +1,7 @@
+---
+deprecations:
+  - |
+    Using APT keys as list of GnuPG ids (via ``apt_keys`` template override) is
+    deprecated since Wallaby release and support for them will be dropped in
+    Xena cycle. If you used this in your template overrides then please migrate
+    into ``remote_apt_keys`` + entries in ``repos.yaml`` file.