Commit 3: Remove \U unicode-escape residue; keep only ECMAScript \uHHHH

\U is a Ruby/PCRE-ism, not ECMAScript. Restrict
RegExpEscape.isUnicode()/getUnicode() to lowercase \uHHHH only. No \U
path exists in ParseRegExp.qll's escapedCharacter, and no corpus case
references \U. .expected files unchanged.

Bundle: github/codeql-action codeql-bundle-v2.26.1 (CodeQL CLI 2.26.1).

Author: Copilot

cpp/ql/lib/semmle/code/cpp/regex/RegexTreeView.qll

@@ -640,7 +640,7 @@ private module Impl implements RegexTreeViewSig {
     /**
      * Holds if this is a unicode escape.
      */
-    private predicate isUnicode() { this.getText().prefix(2) = ["\\u", "\\U"] }
+    private predicate isUnicode() { this.getText().prefix(2) = "\\u" }
 
     /**
      * Gets the unicode char for this escape.