bazel: Run tests on JDK 21

Locally, we compile against and run tests on JDK 21, which gives us
the best performance as well as the ability to (conditionally) use newer
APIs without having to resort to reflection.

.bazelrc

@@ -24,17 +24,16 @@ build:macos --crosstool_top=@local_config_apple_cc//:toolchain
 build:macos --host_crosstool_top=@local_config_apple_cc//:toolchain
 
 # Java
-# Always build for Java 8, even with a newer JDK. This ensures that all
-# artifacts we release are compatible with Java 8 runtimes.
+# Always build for Java 8, even with a newer JDK. This ensures that all artifacts we release are
+# bytecode compatible with Java 8 runtimes. Runtime compatibility (e.g not using APIs that aren't
+# available in Java 8) is verified in CI.
 # Note: We would like to use --release, but can't due to
-#  https://bugs.openjdk.org/browse/JDK-8214165.
-build --javacopt=-target --javacopt=8
+# https://bugs.openjdk.org/browse/JDK-8214165.
 build --java_language_version=8
-build --tool_java_language_version=8
-# Use a hermetic JDK to compile Java code, which also means that contributors
-# don't need to install a JDK to compile Jazzer.
-build --java_runtime_version=remotejdk_17
-build --tool_java_runtime_version=remotejdk_17
+
+# Tools used during the build do not have compatibility requirements, so we can use the most recent
+# language features.
+build --tool_java_language_version=21
 
 # Targeting Java 8 with JDK 21 is deprecated and results in this warning:
 # warning: [options] source value 8 is obsolete and will be removed in a future release
@@ -45,7 +44,13 @@ build --tool_java_runtime_version=remotejdk_17
 build --javacopt=-Xlint:-options
 build --host_javacopt=-Xlint:-options
 
+# Compile for a hermetic JDK so that devs don't have to have a particular JDK installed and
+# configured locally.
+build --java_runtime_version=remotejdk_21
+build --tool_java_runtime_version=remotejdk_21
+
 # Speed up Java compilation by removing indirect deps from the compile classpath.
+# Only has an effect when used together with --disk_cache or --remote_cache.
 build --experimental_java_classpath=bazel
 
 # Android