add workflow to check for jdk updates

.github/check-jdk-updates.yml

@@ -0,0 +1,64 @@
+name: Checks JDK version for minor updates
+
+on:
+  schedule:
+    - cron: '0 0 1 * *' # run once a month at the first day of month
+
+env:
+  JDK_VERSION: 21.0.0
+  JDK_VENDOR: zulu
+
+jobs:
+  jdk-current:
+    name: Check out current version
+    runs-on: ubuntu-latest
+    outputs:
+      jdk-date: ${{ steps.get-data.outputs.jdk-date}}
+    steps:
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 21.0.0
+          distribution: zulu
+          check-latest: false
+      - name: Read JAVA_VERSION_DATE and store in env variable
+        id: get-data
+        run: |
+          date=$(cat ${JAVA_HOME}/release | grep "JAVA_VERSION_DATE=\"" | awk -F'=' '{print $2}' | tr -d '"')
+          echo "jdk-date=${date}" >> "$GITHUB_OUTPUT"
+  jdk-latest:
+    name: Checkout latest jdk version
+    runs-on: ubuntu-latest
+    outputs:
+      jdk-date: ${{ steps.get-data.outputs.jdk-date}}
+      jdk-version: ${{ steps.get-data.outputs.jdk-version}}
+    steps:
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 21
+          distribution: zulu
+          check-latest: true
+      - name: Read JAVA_VERSION_DATE and store in env variable
+        id: get-data
+        run: |
+          date=$(cat ${JAVA_HOME}/release | grep "JAVA_VERSION_DATE=\"" | awk -F'=' '{print $2}' | tr -d '"')
+          echo "jdk-date=${date}" >> "$GITHUB_OUTPUT"
+          version=$(cat ${JAVA_HOME}/release | grep "JAVA_RUNTIME_VERSION=\"" | awk -F'=' '{print $2}' | tr -d '"')
+          echo "jdk-version=${version}" >> "$GITHUB_OUTPUT"
+  notify:
+    name: Notifies for jdk update
+    runs-on: ubuntu-latest
+    needs: [jdk-current, jdk-latest]
+    if: ${{ needs.jdk-latest.outputs.jdk-date }} > ${{ needs.jdk-current.outputs.jdk-date }}
+    steps:
+      - name: Slack Notification
+        uses: rtCamp/action-slack-notify@v2
+        env:
+          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK_URL }}
+          SLACK_USERNAME: 'Cryptobot'
+          SLACK_ICON: false
+          SLACK_ICON_EMOJI: ':bot:'
+          SLACK_CHANNEL: 'cryptomator-desktop'
+          SLACK_TITLE: "JDK update"
+          SLACK_MESSAGE: "Cryptomator-CI JDK can be upgraded to ${{ needs.jdk-latest.outputs.jdk-version }}"
+          SLACK_FOOTER: false
+          MSG_MINIMAL: true