Configure java 21 in bcr presubmits using an anonymous module.

PiperOrigin-RevId: 791661966

Author: jDramaix

.bcr/presubmit.yml

@@ -8,5 +8,10 @@ tasks:
   run_tests:
     platform: ${{ platform }}
     bazel: ${{ bazel }}
+    build_flags:
+      - "--java_language_version=21"
+      - "--java_runtime_version=21"
+      - "--tool_java_language_version=21"
+      - "--tool_java_runtime_version=21"
     build_targets:
       - '@elemental2//:all'