[CI] Replace mergify with automerge action

.github/labeler.yml

@@ -0,0 +1,12 @@
+version: 1
+labels:
+  - label: "automerge"
+    authors: ["softwaremill-ci"]
+    files:
+      - "build.sbt"
+      - "project/plugins.sbt"
+  - label: "dependency"
+    authors: ["softwaremill-ci"]
+    files:
+      - "build.sbt"
+      - "project/plugins.sbt"

.github/workflows/ci.yml

@@ -52,6 +52,44 @@ jobs:
       - name: Check MiMa # disable for major releases
         run: sbt -v core3/mimaReportBinaryIssues
 
+  # `automerge` label is attached iff there is exactly one file changed by steward and this file belongs to a
+  # whitelist specified by `labeler.yml`
+  label:
+    name: Attach automerge label
+    # only for PRs by softwaremill-ci
+    if: github.event.pull_request.user.login == 'softwaremill-ci'
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
+      # count number of files changed
+      - name: Count number of files changed
+        id: count-changed-files
+        run: |
+               N=$(git diff --name-only -r HEAD^1 HEAD | wc -w)
+               echo "changed_files_num=$N" >> $GITHUB_OUTPUT
+      - name: Launch labeler
+        # skip if more than one file changed
+        if: steps.count-changed-files.outputs.changed_files_num == 1
+        uses: srvaroa/labeler@master
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+
+  auto-merge:
+    name: Auto merge
+    # only for PRs by softwaremill-ci
+    if: github.event.pull_request.user.login == 'softwaremill-ci'
+    needs: [ ci, mima, label ]
+    runs-on: ubuntu-22.04
+    steps:
+      - id: automerge
+        name: automerge
+        uses: "pascalgn/automerge-action@v0.16.3"
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+          MERGE_METHOD: "squash"
+
   publish:
     name: Publish release
     needs: [ci]