Checkout 2.3.x branch in CI workflow

Author: eleftherias

.github/workflows/continuous-integration-workflow-2.3.x.yml

@@ -25,6 +25,8 @@ jobs:
       fail-fast: false
     steps:
       - uses: actions/checkout@v2
+        with:
+          ref: '2.3.x'
       - name: Set up JDK ${{ matrix.jdk }}
         uses: actions/setup-java@v1
         with:
@@ -47,6 +49,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          ref: '2.3.x'
       - name: Set up JDK
         uses: actions/setup-java@v1
         with:
@@ -74,6 +78,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+        with:
+          ref: '2.3.x'
       - name: Set up JDK
         uses: actions/setup-java@v1
         with: