Revert "Switch to JDK 21"

This reverts commit 5a2e36b7550639a21808f0e55eb9710f1dd53385.

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

Change-Id: I153c4933f6891c7fff43e522a4d128acad6b6d47
diff --git a/ui/build/config.go b/ui/build/config.go
index a380157..e29d239 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -387,21 +387,22 @@
 
 	// Configure Java-related variables, including adding it to $PATH
 	java8Home := filepath.Join("prebuilts/jdk/jdk8", ret.HostPrebuiltTag())
+	java17Home := filepath.Join("prebuilts/jdk/jdk17", ret.HostPrebuiltTag())
 	java21Home := filepath.Join("prebuilts/jdk/jdk21", ret.HostPrebuiltTag())
 	javaHome := func() string {
 		if override, ok := ret.environ.Get("OVERRIDE_ANDROID_JAVA_HOME"); ok {
 			return override
 		}
+		if ret.environ.IsEnvTrue("EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN") {
+			return java21Home
+		}
 		if toolchain11, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN"); ok && toolchain11 != "true" {
-			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN is no longer supported. An OpenJDK 21 toolchain is now the global default.")
+			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN is no longer supported. An OpenJDK 11 toolchain is now the global default.")
 		}
 		if toolchain17, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK17_TOOLCHAIN"); ok && toolchain17 != "true" {
-			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK17_TOOLCHAIN is no longer supported. An OpenJDK 21 toolchain is now the global default.")
+			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK17_TOOLCHAIN is no longer supported. An OpenJDK 17 toolchain is now the global default.")
 		}
-		if toolchain21, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN"); ok && toolchain21 != "true" {
-			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK21_TOOLCHAIN is no longer supported. An OpenJDK 21 toolchain is now the global default.")
-		}
-		return java21Home
+		return java17Home
 	}()
 	absJavaHome := absPath(ctx, javaHome)