Migrate all workflows to GitHub Actions

This commit completes the migration from Concourse to GitHub Actions,
adding the CI variants and release workflows.

See gh-1024

.github/workflows/backport-bot.yml

@@ -18,16 +18,17 @@ jobs:
       pull-requests: write
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
-      - uses: actions/setup-java@v3
+      - name: Check out code
+        uses: actions/checkout@v4
+      - name: Set up Java
+        uses: actions/setup-java@v4
         with:
-          distribution: 'temurin'
-          java-version: '17'
+          distribution: 'liberica'
+          java-version: 17
       - name: Download BackportBot
         run: wget https://github.com/spring-io/backport-bot/releases/download/latest/backport-bot-0.0.1-SNAPSHOT.jar
       - name: Backport
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           GITHUB_EVENT: ${{ toJSON(github.event) }}
-        run: java -jar backport-bot-0.0.1-SNAPSHOT.jar --github.accessToken="$GITHUB_TOKEN" --github.event_name "$GITHUB_EVENT_NAME" --github.event "$GITHUB_EVENT"
-
+        run: java -jar backport-bot-0.0.1-SNAPSHOT.jar --github.accessToken="$GITHUB_TOKEN" --github.event_name "$GITHUB_EVENT_NAME" --github.event "$GITHUB_EVENT"