Update export-search-results.mdx (#737)

Add in note about export results only being available in new UI after
5.8

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Author: morgangauth

docs/admin/how-to/export-search-results.mdx

@@ -2,7 +2,7 @@
 
 You can export search results to a CSV file by pressing the 'Export results' button in the `Actions` menu above the search results.
 
-For versions before 4.0, view the legacy docs at [sourcegraph/sourcegraph-search-export](https://github.com/sourcegraph/sourcegraph-search-export#sourcegraph-search-results-csv-export-extension)
+<Callout type="note" title="Note">For versions before 5.8, the 'Export Results' action is only available in the legacy UI. For versions before 4.0, view the legacy docs at [sourcegraph/sourcegraph-search-export](https://github.com/sourcegraph/sourcegraph-search-export#sourcegraph-search-results-csv-export-extension).</Callout>
 
 ## FAQs
 
@@ -13,4 +13,4 @@ Before Sourcegraph 4.4.0, the search result export feature used the GraphQL API
 This is expected, as all instances that match for a single file will be listed in the same entry column under the Search matches row. 
 
 #### Network Error when downloading CSV
-Check the browser's dev tools network tab for more details on the specific network error. The CSV file likely exceeds the browser's limit for data URI size. You can limit the size of search results exported with the `count:` filter in the search query.
+Check the browser's dev tools network tab for more details on the specific network error. The CSV file likely exceeds the browser's limit for data URI size. You can limit the size of search results exported with the `count:` filter in the search query.