[MM-61969] Fix regression where search suggestion would break when using uppercase letters (#29400)

webapp/channels/src/components/new_search/search_box.tsx

@@ -144,7 +144,7 @@ const SearchBox = forwardRef(
                 const caretPosition = getCaretPosition();
                 const extraSpace = caretPosition === searchTerms.length ? ' ' : '';
                 setSearchTerms(
-                    searchTerms.slice(0, caretPosition).replace(new RegExp(escapedMatchedPretext + '$'), '') +
+                    searchTerms.slice(0, caretPosition).replace(new RegExp(escapedMatchedPretext + '$', 'i'), '') +
                     value +
                     extraSpace +
                     searchTerms.slice(caretPosition),