CI: Assign milestone on merged PRs (OSGeo#4414)

* CI: Create a workflow to assign milestones

* Add GH_TOKEN for gh cli

* CI: Add GH_REPO for gh cli

* CI: View PR from gh cli using html_url

* CI: Download and parse version file

* CI: Show version file

* CI: Download and parse version file using a pipe

* Show version file output

Clean up

* Pipe version file env to head

* Rename variables to milestone and title

* Edit PR with milestone

* Get milestone from gh cli

* Add comment on why API call is used for getting milestone

* Show if the PR has a milestone set or not

* Do not run steps if a milestone is already set

* Add pull_request_target closed trigger

* Add ref as url parameter

* Remove debugging steps

* CI: Handle RC followed by numbers in sed pattern replacement

.github/workflows/milestones.yml

@@ -0,0 +1,70 @@
+---
+name: Assign Milestone
+
+on:
+  pull_request_target:
+    types: [closed]
+
+jobs:
+  assign-milestone:
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.merged
+    steps:
+      # Retreiving the current milestoone from API instead of github context,
+      # so up-to-date information is used when running after being queued or for reruns
+      # Otherwise, the information should be available using
+      # ${{ github.event.pull_request.milestone.title }}
+      - name: Get current milestone title
+        id: current-milestone
+        run: |
+          echo "milestone<<EOF" >> "${GITHUB_OUTPUT}"
+          gh pr view ${{ github.event.pull_request.html_url }} --json milestone \
+            --jq .milestone.title >> "${GITHUB_OUTPUT}"
+          echo 'EOF' >> "${GITHUB_OUTPUT}"
+        env:
+          GH_TOKEN: ${{ github.token }}
+          GH_REPO: ${{ github.repository }}
+      - name: PR already has a milestone
+        run: echo "PR already has a milestone"
+        if: ${{ steps.current-milestone.outputs.milestone }}
+      - name: PR does not have a milestone
+        run: echo "PR does not have a milestone"
+        if: ${{ !steps.current-milestone.outputs.milestone }}
+      - name: Get VERSION file
+        if: ${{ !steps.current-milestone.outputs.milestone }}
+        id: version-file
+        run: |
+          echo "version<<EOF" >> "${GITHUB_OUTPUT}"
+          gh api \
+            -H "Accept: application/vnd.github.raw" \
+            -H "X-GitHub-Api-Version: 2022-11-28" \
+            "/repos/{owner}/{repo}/contents/include/VERSION?ref=${{ github.sha }}" >> "${GITHUB_OUTPUT}"
+          echo "EOF" >> "${GITHUB_OUTPUT}"
+        env:
+          GH_TOKEN: ${{ github.token }}
+          GH_REPO: ${{ github.repository }}
+      - name: Show version file
+        if: ${{ !steps.current-milestone.outputs.milestone }}
+        run: echo "${VERSIONFILE}"
+        env:
+          VERSIONFILE: ${{ steps.version-file.outputs.version }}
+      - name: Get milestone title from VERSION file
+        if: ${{ !steps.current-milestone.outputs.milestone }}
+        id: milestone
+        run: |
+          version=$(echo "$VERSIONFILE" | head -n 3 | xargs | sed 's/ /./g; s/\(RC[0-9]*\|dev\)//g')
+          echo "title=$version" >> "${GITHUB_OUTPUT}"
+        env:
+          VERSIONFILE: ${{ steps.version-file.outputs.version }}
+      - name: Show milestone title
+        if: ${{ !steps.current-milestone.outputs.milestone }}
+        run: echo "${MILESTONE}"
+        env:
+          MILESTONE: ${{ steps.milestone.outputs.title }}
+      - name: Set PR milestone
+        if: ${{ !steps.current-milestone.outputs.milestone }}
+        run: gh pr edit ${{ github.event.pull_request.html_url }} --milestone "${MILESTONE}"
+        env:
+          GH_TOKEN: ${{ github.token }}
+          GH_REPO: ${{ github.repository }}
+          MILESTONE: ${{ steps.milestone.outputs.title }}