fix(deps): update pre-commit hook davidanson/markdownlint-cli2 to v0.21.0

mentioned in merge request ucam-minimal-gcp-monitoring!65 (merged)

mentioned in merge request gke-cluster!139 (merged)

mentioned in merge request gcp-function!117 (merged)

mentioned in merge request gcp-alert-snooze!145 (merged)

mentioned in merge request ucam-observe-remote-server!102 (merged)

mentioned in merge request gcp-monitoring!71 (merged)

mentioned in merge request ucam-gcp-secret!115 (merged)

mentioned in merge request gcp-scheduled-script!132

mentioned in merge request gcp-container-image-mirror!111 (merged)

mentioned in merge request gcp-alert-to-gitlab-issue!51 (merged)

mentioned in merge request gcp-cloud-run-app!167 (merged)

mentioned in merge request ucam-faas-long!113 (merged)

mentioned in merge request gcp-secret-manager!116 (merged)

mentioned in merge request gcp-notification-to-gitlab-issue!132 (merged)

mentioned in merge request gcp-pubsub-to-ms-teams!140 (merged)

mentioned in merge request ucam-faas!159 (merged)

mentioned in merge request uis-traffic-manager-mtls!39

mentioned in merge request gcp-site-monitoring!147 (merged)

mentioned in merge request gcp-cloud-run-function!86 (merged)