Use Pull Request number instead of ID

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/issues/65139 we introduced pipelines for GitHub pull requests.

In the previous implementation we stored pull_request.id which is an internal ID from GitHub. Instead we should use pull_request.number.

In this MR we fix that.

Thanks @dosuken123 for spotting this.

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content