Reduce limit of contributors so as not to use as many GraphQL points

https://github.com/adobe/gatsby-source-github-file-contributors/blob/5bee0b9d250af5a236df46d05f03f9bd72442ecc/src/gql.js#L58

Right now we take the first 100, to reduce it to the "Last 5 contributors", in case of a frequent contributor showing up only, so that we could fulfill the "Last 5 Contributors" requirement .

Reducing this limit to a lower value will potentially reduce the GraphQL points for a Github API call, and this will help with a very large site -- at the expense of the "Last 5 Contributors" requirement above.

Start with src/gql.js at line 58 and inspect how the contributor limit is used in the GitHub GraphQL query. Determine a lower limit that addresses GraphQL point usage while considering the stated last-five-contributors requirement, then verify the query still returns the intended contributor data.

Written by the indexing model from the issue text.