Add Java 18 build configuration.

Author: jenetics

.github/workflows/gradle.yml

@@ -17,7 +17,7 @@ jobs:
         strategy:
             matrix:
                 os: [ ubuntu-latest, macos-latest ]
-                java-version: [ 17, 18-ea  ]
+                java-version: [ 17, 18  ]
         steps:
             - uses: actions/checkout@v2