[Global Search] Fix convertTagNameToId to use lowercase values (#196819)

## Summary

This PR fixes a bug which caused mixed case tags to not be found in
global search.
Fixes: #196168

The `allTags` argument contains tags with names in the original case
they were created but the `tagName` argument passed in
[search_bar.tsx:180](https://github.com/elastic/kibana/blob/main/x-pack/plugins/global_search_bar/public/components/search_bar.tsx#L180)
is lowercase. Since you can't have tags with the same name but different
casing, converting them to lowercase is safe.

I've also added lowercase conversion to `tagName` argument in case this
function gets called somewhere else and the input is not lowercase.

x-pack/plugins/global_search_bar/public/components/search_bar.tsx

@@ -177,7 +177,7 @@ export const SearchBar: FC<SearchBarProps> = (opts) => {
         let tagIds: string[] | undefined;
         if (taggingApi && rawParams.filters.tags) {
           tagIds = rawParams.filters.tags.map(
-            (tagName) => taggingApi.ui.getTagIdFromName(tagName.toLowerCase()) ?? UNKNOWN_TAG_ID
+            (tagName) => taggingApi.ui.getTagIdFromName(tagName) ?? UNKNOWN_TAG_ID
           );
         } else {
           tagIds = undefined;

x-pack/plugins/saved_objects_tagging/public/utils.ts

@@ -45,7 +45,7 @@ export const getTagsFromReferences = (references: SavedObjectReference[], allTag
 };
 
 export const convertTagNameToId = (tagName: string, allTags: Tag[]): string | undefined => {
-  const found = allTags.find((tag) => tag.name === tagName);
+  const found = allTags.find((tag) => tag.name.toLowerCase() === tagName.toLowerCase());
   return found?.id;
 };