Improve search - Make less API calls

**Is your feature request related to a problem? Please describe.**
The search functionality does not work for our content. Firstly it is very slow to use, and secondly after 2 searches it fails becuase it hits the GitHub API limit. This could be fixed by changing the code to use the GitHub API search endpoint.

see https://answers.netlify.com/t/cms-search-hitting-github-api-limit/39124

**Describe the solution you'd like**
The search code could be optimised so it doesn't make a request for every item in the collection.
This should be possible with the GitHub search endpoint - https://docs.github.com/en/rest/reference/search#search-code

**Describe alternatives you've considered**
We have turned off search to avoid the problem, by using the `search` config option.
https://www.netlifycms.org/docs/configuration-options/#search

**Additional context**
#5501
https://github.com/netlify/netlify-cms/issues/5501#issuecomment-861354125