feat: Implement multi-JDK CI build strategy

- Add Java 21 job to test core library compatibility
- Add Java 24 job to test full project including API tracker
- Separate cache keys for different JDK versions
- Validates Java 21 promise while enabling Java 24 tooling

Closes #9

Author: simbo1905

.github/workflows/maven.yml

@@ -8,10 +8,33 @@ on:
     types: [opened, synchronize, reopened]
 
 jobs:
-  build:
+  test-java21:
+    name: Test Core Library (Java 21)
     runs-on: ubuntu-latest
-    name: Build with JDK 24
+    
+    steps:
+    - uses: actions/checkout@v4
+    
+    - name: Set up JDK 21
+      uses: actions/setup-java@v4
+      with:
+        java-version: '21'
+        distribution: 'oracle'
+    
+    - name: Cache Maven dependencies
+      uses: actions/cache@v4
+      with:
+        path: ~/.m2
+        key: ${{ runner.os }}-m2-java21-${{ hashFiles('**/pom.xml') }}
+        restore-keys: ${{ runner.os }}-m2-java21
+    
+    - name: Test core library (Java 21)
+      run: mvn clean test -pl json-java21
 
+  test-java24:
+    name: Test with Tooling (Java 24)
+    runs-on: ubuntu-latest
+    
     steps:
     - uses: actions/checkout@v4
 
@@ -25,11 +48,8 @@ jobs:
       uses: actions/cache@v4
       with:
         path: ~/.m2
-        key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
-        restore-keys: ${{ runner.os }}-m2
-    
-    - name: Build with Maven
-      run: mvn clean compile
+        key: ${{ runner.os }}-m2-java24-${{ hashFiles('**/pom.xml') }}
+        restore-keys: ${{ runner.os }}-m2-java24
 
-    - name: Package JAR
-      run: mvn package
+    - name: Test full project (Java 24)
+      run: mvn clean test