Replace to(Upper|Lower)Case with Ascii.to$1Case

The presubmits are warning that toUpperCase() and toLowerCase() are locale-specific, and advise using Ascii.toUpperCase() and Ascii.toLowerCase() as a local-invariant alternative.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127583677

java/google/registry/tools/CommandUtilities.java

@@ -14,6 +14,7 @@
 
 package google.registry.tools;
 
+import com.google.common.base.Ascii;
 import com.google.common.base.Strings;
 
 /** Container class for static utility methods. */
@@ -25,6 +26,6 @@ class CommandUtilities {
 
   /** Prompts for yes/no input using promptText, defaulting to no. */
   static boolean promptForYes(String promptText) {
-    return System.console().readLine(promptText + " (y/N): ").toUpperCase().startsWith("Y");
+    return Ascii.toUpperCase(System.console().readLine(promptText + " (y/N): ")).startsWith("Y");
   }
 }