Reduce matrix job to save on GitHub resources

.github/workflows/upgrade.yml

@@ -1,19 +1,19 @@
-# Portability testing across OSes and newer Java versions
+# Portability testing across OSes and Java LTS versions
 # ---
 # Regularly check if we remain compatible with other OSes than Linux and if we
-# could upgrade to newer versions of Java.
+# could upgrade to newer LTS versions of Java.
 #
 # This acts more like an indicator if something is going to break. Our main
 # target platform is Linux and we build against it during our regular workflow
-# runs. Likewise, we're just testing if we could support newer version of
-# Java. We're still taking very deliberate decisions to move to newer (LTS)
-# versions of Java.
-name: 'Portability testing OSes and Java versions'
+# runs. Likewise, we're just testing if we could support newer LTS version of
+# Java. We're still taking very deliberate decisions to upgrade to the next
+# LTS version of Java.
+name: 'Portability testing OSes and Java LTS versions'
 
 on:
   schedule:
-    # runs every Monday at 5:00
-    - cron: '0 5 * * 1'
+    # runs at 5:00 on the first of every month
+    - cron: '0 5 1 * *'
   # can be triggered manually
   workflow_dispatch:
 
@@ -22,19 +22,19 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: [ "18", "19", "20", "21", "22" ]
+        java-lts: [ '17', '21' ]
         os: [ ubuntu-latest, macos-latest, windows-latest ]
     runs-on: ${{ matrix.os }}
     continue-on-error: true
     name: 'Building with Java ${{ matrix.version }} on ${{ matrix.os }}'
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-      - name: Setup Java ${{ matrix.version }}
+      - name: Setup Java ${{ matrix.java-lts }}
         uses: actions/setup-java@v4
         with:
           distribution: "temurin"
-          java-version: ${{ matrix.version }}
+          java-version: ${{ matrix.java-lts }}
       - name: Setup Gradle
         uses: gradle/actions/setup-gradle@v3
       - name: Build and Test