Detect if pipeline runs for a GitHub pull request

When using a mirror for CI/CD only we register a pull_request
webhook. When a pull_request webhook is received, if the
source branch SHA matches the actual head of the branch in the
repository we create immediately a new pipeline for the
external pull request. Otherwise we store the
pull request info for when the push webhook is received.

When using "only/except: external_pull_requests" we can detect
if the pipeline has a open pull request on GitHub and create or
not the job based on that.

Update security dashboards documentation

See merge request gitlab-org/gitlab-ce!32674

Reflect the addition of a "hide dismissed vulnerabilities" toggle in
security dashboards, see https://gitlab.com/gitlab-org/gitlab-ee/issues/9102

Add .gitlab/ci/frontend.gitlab-ci.yml to frontend review roulette

See merge request gitlab-org/gitlab-ce!32466

Adjust routable metric

See merge request gitlab-org/gitlab-ce!32591

Avoid checking dns rebind protection in validation

Closes #66723

See merge request gitlab-org/gitlab-ce!32577

Change serveo link as it often goes down

See merge request gitlab-org/gitlab-ce!32680