Adjust thresholds for several endpoints

Updating thresholds based on latest improvements and degradations

• gitlab-org/gitlab#338359 (closed), gitlab-org/gitlab#348802, gitlab-org/gitlab#371965
• https://10k.testbed.gitlab.net/-/grafana/d/jH6yoWVMz/gitlab-performance-sitespeed-results?orgId=1&from=now-90d&to=now&var-test_name=web_group&var-test_name=web_project&var-test_name=web_project_file_rendered&var-test_name=web_project_file_source&var-test_name=web_project_issues