Adjust LCP and TBT thresholds per review

Updating LCP and TBT thresholds based on historical results test analysis. All relevant new performance issues were raised.

• https://10k.testbed.gitlab.net/-/grafana/d/jH6yoWVMz/gitlab-performance-sitespeed-results?orgId=1&var-test_name=All&from=now-90d&to=now

Related to https://gitlab.com/gitlab-org/quality/quality-engineering/team-tasks/-/issues/2828+