Add Postgres 16 to nightly rspec builds

What does this MR do and why?

Adds Postgres 16 to nightly test runs.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #425470 (closed)

.gitlab/ci/rails.gitlab-ci.yml

@@ -958,27 +958,32 @@ rspec migration pg16:
   extends:
     - .rspec-base-pg16
     - .rspec-base-migration
+    - .rails:rules:default-branch-schedule-nightly--code-backstage
     - .rspec-migration-parallel
 
 rspec background_migration pg16:
   extends:
     - .rspec-base-pg16
     - .rspec-base-migration
+    - .rails:rules:default-branch-schedule-nightly--code-backstage
     - .rspec-background-migration-parallel
 
 rspec unit pg16:
   extends:
     - .rspec-base-pg16
+    - .rails:rules:default-branch-schedule-nightly--code-backstage
     - .rspec-unit-parallel
 
 rspec integration pg16:
   extends:
     - .rspec-base-pg16
+    - .rails:rules:default-branch-schedule-nightly--code-backstage
     - .rspec-integration-parallel
 
 rspec system pg16:
   extends:
     - .rspec-base-pg16
+    - .rails:rules:default-branch-schedule-nightly--code-backstage
     - .rspec-system-parallel
 # EE/FOSS: default branch nightly scheduled jobs #
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