Updates .gitlab/ci scripts on 12-4-stable

What does this MR do?

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8727#note_262807975

Updates .gitlab/ scripts on 12-4-ee-stable. The following commits were cherry-picked:

• Don't run Review App jobs for backstage changes - Had to be picked otherwise the in preparation for the next commit
• Improve the only/except extendable jobs
• Allow failure for schedule:package-and-qa due to failure rate

Additionally, I had to modify the scripts to fix some failures. These modifications are wrapped on 76c02c45

Test MR on Security https://gitlab.com/gitlab-org/security/gitlab/merge_requests/15

.gitlab/ci/docs.gitlab-ci.yml

@@ -2,7 +2,7 @@
   extends:
     - .default-tags
     - .default-retry
-    - .only-docs-changes
+    - .only:changes-docs
   only:
     refs:
       - merge_requests
@@ -50,7 +50,7 @@ docs lint:
     - .default-tags
     - .default-retry
     - .default-only
-    - .only-docs-changes
+    - .only:changes-docs
   image: "registry.gitlab.com/gitlab-org/gitlab-build-images:gitlab-docs-lint"
   stage: test
   dependencies: []
@@ -76,7 +76,7 @@ graphql-docs-verify:
     - .default-cache
     - .default-only
     - .default-before_script
-    - .only-graphql-changes
+    - .only:changes-graphql
   variables:
     SETUP_DB: "false"
   stage: test