Make hidden files warning use GlAlert

What does this MR do and why?

For #353429 (closed)

This MR replaces a div.alert.alert-warning with a <gl-alert> in diffs/components/hidden_files_warning.vue

Screenshots or screen recordings

before	after

How to set up and validate locally

1. git checkout 353429-migrate-hidden-files-warning
2. open a merge request
3. change lots of files in your merge request and/or apply this patch to make the alert show up:

diff --git a/app/assets/javascripts/diffs/components/app.vue b/app/assets/javascripts/diffs/components/app.vue
index 5707e4d67f9..6fd6ca61b2f 100644
--- a/app/assets/javascripts/diffs/components/app.vue
+++ b/app/assets/javascripts/diffs/components/app.vue
@@ -623,7 +623,7 @@ export default {
 
       <template v-if="!isBatchLoadingError">
         <hidden-files-warning
-          v-if="visibleWarning == $options.alerts.ALERT_OVERFLOW_HIDDEN"
+          v-if="true"
           :visible="numVisibleFiles"
           :total="numTotalFiles"
           :plain-diff-path="plainDiffPath"

4. navigate to the Changes tab of your merge request

MR acceptance checklist

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