Chore: Add PR Check action enforcing backport decision (#43916)

Ref https://github.com/grafana/grafana-github-actions/pull/53
Ref https://github.com/grafana/grafana-release/issues/60

.github/pr-checks.json

@@ -5,5 +5,14 @@
     "targetUrl": "https://github.com/grafana/grafana/blob/main/contribute/merge-pull-request.md#assign-a-milestone",
     "success": "Milestone set",
     "failure": "Milestone not set"
+  },
+  {
+    "type": "check-backport",
+    "title": "Backport Check",
+    "backportEnabled": "Backport enabled",
+    "backportSkipped": "Backport skipped",
+    "failure": "Backport decision needed",
+    "targetUrl": "https://github.com/grafana/grafana/blob/main/contribute/merge-pull-request.md#should-the-pull-request-be-backported",
+    "skipLabels": [ "backport", "no-backport"]
   }
 ]

.github/workflows/pr-checks.yml

@@ -4,6 +4,8 @@ on:
     types:
       - opened
       - synchronize
+      - labeled
+      - unlabeled
   issues:
     types:
       - milestoned

contribute/merge-pull-request.md

@@ -85,8 +85,21 @@ In case the pull request introduces a breaking change you should document this.
 
 ### Should the pull request be backported?
 
+An active decision of backporting needs to be taken for every pull request. There's a pull request check named **Backport Check** that will enforce this. By adding/removing labels on the pull request the check will be re-evaluated.
+
+#### No backport
+
+If you don't want to backport you need to add a label named **no-backport** to the pull request.
+
+#### Backport
+
 If your pull request has changes that need to go into one or several existing release branches you need to backport the changes. Please refer to [Backport PR](.github/bot.md#backport-pr) for detailed instructions.
 
+The general rule of thumb regarding what changes goes into what release is:
+
+- bug fixes should be released in patch releases, e.g. v8.1.3, if the bug was introduced in the same major/minor or lower patch version.
+- new features should go into the next major/minor version, e.g. v8.0.0, v8.2.0.
+
 Some examples when backport is required:
 
 - The change needs to be released in the next upcoming patch release, e.g. v8.1.3, so you have to backport it, e.g. into the v8.1.x release branch.