Java21のビルドを追加

Author: irof

.github/workflows/build-main.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest, windows-latest ]
-        java-version: [ '17' ]
+        java-version: [ '17', '21' ]
     name: build Java ${{ matrix.java-version }} (${{ matrix.os }})
     env:
       GRADLE_OPTS: '-Dorg.gradle.daemon=false -Dfile.encoding=UTF-8'