Upgrade default `java_toolchain.java_runtime` to JDK 21

PiperOrigin-RevId: 570684954
Change-Id: Id3fd0f2c30aaacf0e69f46c0c6927a26c2ef0638

toolchains/default_java_toolchain.bzl

diff --git a/toolchains/default_java_toolchain.bzl b/toolchains/default_java_toolchain.bzl
index 788f91d..8e28187 100644
--- a/toolchains/default_java_toolchain.bzl
+++ b/toolchains/default_java_toolchain.bzl
@@ -92,7 +92,7 @@
     reduced_classpath_incompatible_processors = [
         "dagger.hilt.processor.internal.root.RootProcessor",  # see b/21307381
     ],
-    java_runtime = Label("//toolchains:remotejdk_17"),
+    java_runtime = Label("//toolchains:remotejdk_21"),
 )
 
 DEFAULT_TOOLCHAIN_CONFIGURATION = _BASE_TOOLCHAIN_CONFIGURATION