ci: Add milestone workflow (#3887)

This is a GitHub Actions based alternative to the milestone bot that we run. The actions setup will

- When a PR is opened or reopened, and has no milestone associated, it will assign the `next` milestone
- When a PR is milestoned or demilestoned, it will add a status to the PR like the current bot does it.

This should allow us to require a milestone to be set before merging, without having to host the bot anymore.

.github/workflows/milestone.yml

@@ -0,0 +1,58 @@
+name: Check PR milestone
+
+on:
+  pull_request_target:
+    types: [milestoned, demilestoned, opened, reopened]
+    branches:
+      - main
+
+jobs:
+  check_milestone:
+    if: ${{ github.event.issue.pull_request || github.event.pull_request }}
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: "Check for milestone on PR"
+        uses: actions/github-script@v7
+        with:
+          script: |
+            let milestone = context.payload.pull_request.milestone;
+
+            if(context.payload.action === 'opened' || context.payload.action === 'reopened') {
+              if(milestone !== null) {
+                  core.notice(`Milestone is ${milestone.title}`);
+              } else {
+                const milestones = await github.rest.issues.listMilestones({
+                  owner: context.repo.owner,
+                  repo: context.repo.repo,
+                  state: "open"
+                });
+                for (const default_milestone of milestones.data) {
+                  if (default_milestone.title === "next") {
+                    core.notice(`No milestone set, setting default milestone: ${default_milestone.title}`);
+
+                    await github.rest.issues.update({
+                      owner: context.repo.owner,
+                      repo: context.repo.repo,
+                      issue_number: context.issue.number,
+                      milestone: default_milestone.number
+                    });
+                    return;
+                  }
+                }
+
+                core.warning("Could not find default milestone named 'next'");
+
+              }
+
+            }
+            else {
+              if(milestone !== null) {
+                  core.notice(`Milestone is ${milestone.title}`);
+              } else {
+                  core.setFailed("No milestone: Please add a version milestone");
+              }
+            }