Revert^2 "Switch to JDK 21"

This reverts commit b832ae19d8.

Reason for revert: Reland Switch to JDK 21 when related issues are resolved.

Bug: 313924276
Change-Id: Ibb5031d9892d1ddc5bc4d2af9885c3192b8ac8ad
Test: TH

java/config/config.go

@@ -132,12 +132,7 @@ func init() {
 		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")