Revert "Switch to JDK 21"

This reverts commit 5a2e36b755.

Reason for revert: Preparing revert to have it ready in case of breakages relating to using JDK 21.

Change-Id: I153c4933f6891c7fff43e522a4d128acad6b6d47

java/config/config.go

@@ -131,7 +131,12 @@ func init() {
 		if override := ctx.Config().Getenv("OVERRIDE_JLINK_VERSION_NUMBER"); override != "" {
 			return override
 		}
-		return "21"
+		switch ctx.Config().Getenv("EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN") {
+		case "true":
+			return "21"
+		default:
+			return "17"
+		}
 	})
 
 	pctx.SourcePathVariable("JavaToolchain", "${JavaHome}/bin")