Switch to JDK 21

Bug: 313924276
Test: n
Change-Id: Ia9682181a6ce35ed0102d60bf7df1def8f876fd0
diff --git a/java/config/config.go b/java/config/config.go
index 6a945ac..d720046 100644
--- a/java/config/config.go
+++ b/java/config/config.go
@@ -131,12 +131,7 @@
 		if override := ctx.Config().Getenv("OVERRIDE_JLINK_VERSION_NUMBER"); override != "" {
 			return override
 		}
-		switch ctx.Config().Getenv("EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN") {
-		case "true":
-			return "21"
-		default:
-			return "17"
-		}
+		return "21"
 	})
 
 	pctx.SourcePathVariable("JavaToolchain", "${JavaHome}/bin")