fix: github actions now using java 21

Author: myoun

.github/workflows/build.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       matrix:
         # Use these Java versions
-        java: [17]
+        java: [21]
         # and run on both Linux and Windows
         os: [ubuntu-20.04, windows-latest]
     runs-on: ${{ matrix.os }}
@@ -25,7 +25,7 @@ jobs:
       - name: build
         run: ./gradlew build
       - name: capture build artifacts
-        if: ${{ runner.os == 'Linux' && matrix.java == '17' }} # Only upload artifacts built from latest java on one OS
+        if: ${{ runner.os == 'Linux' && matrix.java == '21' }} # Only upload artifacts built from latest java on one OS
         uses: actions/upload-artifact@v2
         with:
           name: Artifacts