Increase pages size for github_organization data source

[j3parker]

We are looking to use the github_organization data source in part to verify that users from config files (maintained by humans and used by our Terraform code) belong to an organization (something we don't manage via TF) before trying to assign them to teams or as collaborators, etc.

I noticed that the github_organization data source takes ~2 minutes to read for one of our orgs. I think it is related to having to page through the list of repositories and users.

This PR changes the page size for both of those calls from 10 to 100 (the maximum). I did a bit of searching in the repo and I didn't see this coming up before. If there is a reason GitHub would prefer small pages that's fine. I'll link to the docs inline with the diff.

cc @kfcampbell