Add monthly release pipeline

🔍 What does this MR do and why?

• Adds a new pipeline for monthly releases, introducing the finalize stage.
• Adds slack notifications to the update protected branches service.

Note this MR does not create the pipeline or add it to the monthly release template. That will be added in !2560 (merged).

Related to gitlab-com/gl-infra/delivery#19546 (closed)

🛠 Testing

To test this, I pushed a modified branch to the ops instance so all slack notifications go to the test channel and the protected branches are not updated since that was tested in a previous MR:

Branch modifications

diff --git a/lib/release_tools/monthly/finalize/update_protected_branches.rb b/lib/release_tools/monthly/finalize/update_protected_branches.rb
index 985920ae..cf3da734 100644
--- a/lib/release_tools/monthly/finalize/update_protected_branches.rb
+++ b/lib/release_tools/monthly/finalize/update_protected_branches.rb
@@ -20,20 +20,20 @@ module ReleaseTools

             next if SharedStatus.dry_run?

-            Retriable.with_context(:api) do
-              client.unprotect_branch(project, old_branch(ee: project.ee_branch?))
-            end
-
-            Retriable.with_context(:api) do
-              client.protect_branch(
-                project,
-                new_branch(ee: project.ee_branch?),
-                merge_access_level: MAINTAINER_ACCESS_LEVEL,
-                allowed_to_merge: allowed_to_merge,
-                allowed_to_push: allowed_to_push,
-                code_owner_approval_required: false
-              )
-            end
+            # Retriable.with_context(:api) do
+            #   client.unprotect_branch(project, old_branch(ee: project.ee_branch?))
+            # end
+
+            # Retriable.with_context(:api) do
+            #   client.protect_branch(
+            #     project,
+            #     new_branch(ee: project.ee_branch?),
+            #     merge_access_level: MAINTAINER_ACCESS_LEVEL,
+            #     allowed_to_merge: allowed_to_merge,
+            #     allowed_to_push: allowed_to_push,
+            #     code_owner_approval_required: false
+            #   )
+            # end
           end

           send_slack_notification(:success)
diff --git a/lib/release_tools/slack/pipeline_notifier.rb b/lib/release_tools/slack/pipeline_notifier.rb
index 8c403086..6150b309 100644
--- a/lib/release_tools/slack/pipeline_notifier.rb
+++ b/lib/release_tools/slack/pipeline_notifier.rb
@@ -17,7 +17,7 @@ module ReleaseTools
         logger.info("Notifying the stage of a #{release_type} pipeline", stage: stage, release_manager: user_name, pipeline_url: pipeline_url)

         ReleaseTools::Slack::Message.post(
-          channel: ReleaseTools::Slack::F_UPCOMING_RELEASE,
+          channel: ReleaseTools::Slack::NOTIFICATION_TESTS,
           message: fallback_message[stage],
           blocks: slack_blocks
         )
diff --git a/lib/release_tools/slack/release_pipeline_notifier.rb b/lib/release_tools/slack/release_pipeline_notifier.rb
index b8e247a2..fb530336 100644
--- a/lib/release_tools/slack/release_pipeline_notifier.rb
+++ b/lib/release_tools/slack/release_pipeline_notifier.rb
@@ -19,7 +19,7 @@ module ReleaseTools
         logger.info('Posting slack message', job_type: job_type, status: status)

         ReleaseTools::Slack::Message.post(
-          channel: ReleaseTools::Slack::F_UPCOMING_RELEASE,
+          channel: ReleaseTools::Slack::NOTIFICATION_TESTS,
           message: fallback_message,
           blocks: slack_blocks
         )

Description	Reference
I ran a pipeline with the MONTHLY_RELEASE_PIPELINE variable
The created pipeline
Pipeline link	https://ops.gitlab.net/gitlab-org/release/tools/-/pipelines/2214697
Slack notifications

🛃 Author Check-list

• [-] Has documentation been updated?