Support experimental building with OpenJDK 21

Use EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN=true to build with OpenJDK 21
while still targeting java language version 17.

Bug: 313924276
Test: m EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN=true
Change-Id: Idc892bb7519e597f1e280ca0765c1a281bb29955

java/config/config.go

@@ -133,7 +133,12 @@ func init() {
 		if override := ctx.Config().Getenv("OVERRIDE_JLINK_VERSION_NUMBER"); override != "" {
 			return override
 		}
-		return "17"
+		switch ctx.Config().Getenv("EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN") {
+		case "true":
+			return "21"
+		default:
+			return "17"
+		}
 	})
 
 	pctx.SourcePathVariable("JavaToolchain", "${JavaHome}/bin")