Tweak issue and MR discussion API thresholds
Adjusting thresholds based on latest results - gitlab-org/gitlab#334435 (comment 1472368581), known issues with issues https://1k.testbed.gitlab.net/-/grafana/d/J0ysCtCWz/gpt-test-results?orgId=1&from=now-30d&to=now&var-test_name=api_v4_projects_issues_issue&var-test_name=api_v4_projects_issues_search&var-test_name=api_v4_projects_merge_requests_merge_request_discussions&var-test_name=api_v4_projects_repository_commits_commit_diff&var-test_name=web_project_file_rendered