Remove label issue and MR counts from default API responses

What does this MR do?

These counts significantly increase the load time for these requests. Users can now opt in to receiving the counts by setting with_counts=true in requests. This is a breaking change, but hopefully a fairly minor one.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/63103.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.