Revert^2 "Switch to JDK 21"

This reverts commit b832ae19d84132ffe8d780009e772f6602d6b027.

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

Bug: 313924276
Change-Id: Ibb5031d9892d1ddc5bc4d2af9885c3192b8ac8ad
Test: TH
diff --git a/ui/build/config.go b/ui/build/config.go
index e29d239..a380157 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -387,22 +387,21 @@
 
 	// 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 11 toolchain is now the global default.")
+			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK11_TOOLCHAIN is no longer supported. An OpenJDK 21 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 17 toolchain is now the global default.")
+			ctx.Fatalln("The environment variable EXPERIMENTAL_USE_OPENJDK17_TOOLCHAIN is no longer supported. An OpenJDK 21 toolchain is now the global default.")
 		}
-		return java17Home
+		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
 	}()
 	absJavaHome := absPath(ctx, javaHome)