CI: Drop Ubuntu 16.04

Ubuntu 16.04 is now fully out of support.

Introduce an 18.04 GCC Debug job in lieu of losing the 16.04 job.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Denis Mukhin <dmukhin@ford.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

automation/gitlab-ci/build.yaml

@@ -617,21 +617,16 @@ fedora-41-x86_64-gcc-debug:
   variables:
     CONTAINER: fedora:41-x86_64
 
-ubuntu-16.04-x86_64-gcc:
-  extends: .gcc-x86-64-build
-  variables:
-    CONTAINER: ubuntu:16.04-x86_64
-
-ubuntu-16.04-x86_64-gcc-debug:
-  extends: .gcc-x86-64-build-debug
-  variables:
-    CONTAINER: ubuntu:16.04-x86_64
-
 ubuntu-18.04-x86_64-gcc:
   extends: .gcc-x86-64-build
   variables:
     CONTAINER: ubuntu:18.04-x86_64
 
+ubuntu-18.04-x86_64-gcc-debug:
+  extends: .gcc-x86-64-build-debug
+  variables:
+    CONTAINER: ubuntu:18.04-x86_64
+
 ubuntu-20.04-x86_64-gcc:
   extends: .gcc-x86-64-build
   variables: