GitHub importer - Add query parameter for pagination limit to GitHub importer API

What does this MR do and why?

This MR removes the operational FF github_importer_lower_per_page_limit and introduces an optional pagination_limit parameter to our GitHub importer API instead as suggested in !67150 (comment 2230534426)

References

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MR acceptance checklist

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Screenshots or screen recordings

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How to set up and validate locally

1. Run a GH import with a pagination_limit set to anything other than the default (which is 100), e.g.:

curl --trace --url "http://localhost:3000/api/v4/import/github" \
  --header "content-type: application/json" \
  --header "PRIVATE-TOKEN: GITLAB_PAT" \
  --data '{
    "personal_access_token": "GITHUB_PAT",
    "repo_id": "GITHUB_REPO_ID",
    "target_namespace": "TARGET_NAMESPACE",
    "new_name": "NEW_NAME",
    "timeout_strategy": "optimistic",
    "pagination_limit": 50,
    "optional_stages": {
      "single_endpoint_notes_import": true,
      "attachments_import": true
    }
}'

Related to #506767 (closed)