Use toolchains for Java 21

See gh-42433

Author: philwebb

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
           - version: 17
             toolchain: false
           - version: 21
-            toolchain: false
+            toolchain: true
           - version: 22
             toolchain: true
           - version: 23