build: Java 21 is required for Minecraft >=1.20

Author: timbru31

.github/workflows/maven.yml

@@ -11,8 +11,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest, macos-13] # OpenJDK 8 is not supported on macos-14+ (M1).
-        java: [8, 11, 17]
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        java: [21]
 
     steps:
     - uses: actions/checkout@v4.2.2