repo-updater: Increase batch when fetching Bitbucket Server repos

Created by: mrnugget

A customer reported performance issues when saving their external service configuration for Bitbucket Server (which contains 10s of thousands of repos)

I successfully replicated the issue locally by generating 20k artificial Bitbucket Server repos in my local Bitbucket server instance and could narrow the problem down to the syncer, which took several minutes to fully fetch all 20k repos.

We're already using batching when fetching BB Server repos, but the batch size is 100. (see 0)

According to the docs (see 1) the maximum batch size is set by the site administrator of the BB Server instance and the the default value for page.max.repositories is 1000 (see 2)

So in this commit we set the default batch size to 1000.

Test plan: manually testing it locally && go test