fix(toc): adjust mobile selector, should always be navbar (#11589)

### Related Ticket(s)

Closes #11587

### Description

Fixes sticky behavior issue with TOC Dotcom shell at less than lg breakpoint.

### Testing instructions

1. Open the Dotcom shell > Default
2. Scroll the page
3. The Masthead should be sticky until it reaches the TOC, then the masthead should scroll away, while the TOC becomes sticky to the top.
4. Repeat at both `lg` and less than `lg` breakpoints
5. Repeat 1-4 with the Dotcom shell > With ToC horizontal  story

### Changelog

**Changed**

- TOC dotcom shell sticky behavior fixed at less than lg breakpoint

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<!-- *** "test: e2e": Codesandbox examples and e2e integration tests -->
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packages/utilities/src/utilities/StickyHeader/StickyHeader.js

@@ -74,21 +74,17 @@ class StickyHeader {
 
     const tocRoot = toc.shadowRoot;
 
-    const desktopSelector = `.${c4dPrefix}-ce--table-of-contents__items-container`;
-
+    this._tableOfContentsInnerBar = tocRoot.querySelector(
+      `.${prefix}--tableofcontents__navbar`
+    );
     if (window.innerWidth > gridBreakpoint) {
       if (toc.layout === 'horizontal') {
-        this._tableOfContentsInnerBar = tocRoot.querySelector(
-          `.${prefix}--tableofcontents__navbar`
-        );
         this._tableOfContentsLayout = 'horizontal';
       } else {
-        this._tableOfContentsInnerBar = tocRoot.querySelector(desktopSelector);
+        this._tableOfContentsInnerBar = tocRoot.querySelector(
+          `.${c4dPrefix}-ce--table-of-contents__items-container`
+        );
       }
-    } else {
-      this._tableOfContentsInnerBar = tocRoot.querySelector(
-        `.${prefix}--tableofcontents__sidebar`
-      );
     }
   }