fix(deps): update module github.com/terraform-docs/terraform-docs to v0.21.0

mentioned in merge request gcp-container-image-mirror!96 (merged)

mentioned in merge request gcp-site-monitoring!128 (merged)

mentioned in merge request gcp-notification-to-gitlab-issue!120

mentioned in merge request gcp-alert-to-gitlab-issue!6 (merged)

mentioned in merge request ucam-faas!138 (merged)

mentioned in merge request gcp-cloud-run-app!155 (merged)

mentioned in merge request gcp-secret-manager!97 (merged)

mentioned in merge request ucam-faas-long!90 (merged)

mentioned in merge request gke-cluster!94 (merged)

mentioned in merge request gcp-function!98 (merged)

mentioned in merge request ucam-minimal-gcp-monitoring!8 (merged)

mentioned in merge request gcp-monitoring!52 (merged)

mentioned in merge request gcp-scheduled-script!113 (merged)

mentioned in merge request ucam-gcp-secret!93 (merged)

mentioned in merge request gcp-cloud-run-function!71 (merged)

mentioned in merge request gcp-alert-snooze!120 (merged)

mentioned in merge request ucam-observe-remote-server!43 (merged)

mentioned in merge request gcp-pubsub-to-ms-teams!115 (merged)