perf: cache regex in StripWhitespace

Use existing Whitespace Lazy<Regex> instead of creating new Regex each call.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/Umbraco.Core/Extensions/StringExtensions.Manipulation.cs

@@ -345,7 +345,7 @@ public static partial class StringExtensions
     /// </summary>
     /// <param name="txt"></param>
     /// <returns></returns>
-    public static string StripWhitespace(this string txt) => Regex.Replace(txt, @"\s", string.Empty);
+    public static string StripWhitespace(this string txt) => Whitespace.Value.Replace(txt, string.Empty);
 
     /// <summary>
     ///     Strips carrage returns and line feeds from the specified text.