[BitbucketServerImport] Reduce pull requests batch size to 50

What does this MR do and why?

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/456416

Problem

PullRequestsImporter can fetch missing commits for closed or merged pull requests. However, it can lead to large requests and raises a Gitaly exception.

Gitlab::Git::ResourceExhaustedError received metadata size exceeds soft limit (10009 vs. 8192)

Solution

Decrease the number of fetches pull requests in the batch.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.