using Java 21 for build workflow

Author: clausnagel

.github/workflows/gml-objects-build.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        java: [17]
+        java: [21]
         distribution: [temurin]
     runs-on: ${{ matrix.os }}
     steps: