ci: add release notifier to pipeline

Configured a Slack message to be sent to Team SRE Foundations when a new
release is ready to be published. (Effectively, after every merge into
`origin/main`.)

This notifier was added to ensure dependency upgrades are actually
released, not just merged and forgotten/unreleased.

.buildkite/pipeline.yaml

@@ -1,7 +1,7 @@
 steps:
   - block: "Release build"
-    branches:
-      - main
+    # branches:
+    #   - main
     fields:
       - select: "Prerelease?"
         key: "release-type"
@@ -32,3 +32,9 @@ steps:
       - cultureamp/aws-sm#v2.2.0:
           env:
             GITHUB_TOKEN: /cfparams/GITHUB_TOKEN
+    notify:
+      - slack:
+          channels:
+            - "#team_sre_foundations_alerts"
+          message: ":fyii: <!subteam^S03CQLT3G2J>: A new version of CFParams is ready to be released :shipit:"
+        # if: build.branch == "main"