chore(operators): remove ARC, MongoDB and OpenSearch operators

[surrealwolf]

Summary

Removes three operators that had no consumers on any cluster: GitHub ARC (gha-runner-scale-set-controller 0.13.1), MongoDB Community operator 0.13.0, and OpenSearch operator 2.8.0.

Already done live (2026-10-05)

• Pre-check on poc/nprd/prd/local: zero custom resources for every actions.github.com, legacy actions.summerwind.dev, mongodbcommunity.mongodb.com and opensearch.opster.io CRD.
• Per cluster (poc → nprd → prd): helm uninstall of the three releases, deleted the remaining ARC/summerwind/MongoDB CRDs (OpenSearch CRDs went with the chart), deleted the actions-runner-system, mongodb and opensearch-operator-system namespaces. No leftover CRDs, ClusterRoles, APIServices or webhooks.
• Terraform: state backed up, then terraform state rm of the 9 null_resource.deploy_{mongodb_community_operator,opensearch_operator,arc}_{nprd,prd,poc}_apps addresses (state 168 → 159). Targeted -refresh=false plan on this branch: No changes. Before the state rm, the same plan showed exactly those 9 destroys and nothing else.

Code changes

• terraform/main.tf: remove the 9 resources (their destroy provisioners were not used: they continue on failure and leave CRDs behind).
• terraform/variables.tf, terraform.tfvars.example: remove opensearch_operator_version, mongodb_operator_version, github_arc_controller_version. Remove these from your local terraform.tfvars too, otherwise Terraform warns about undeclared variables.
• scripts/: remove ARC/GitHub App helpers (install-github-arc.sh, setup-github-app-arc.sh, complete-arc-setup.sh, create-github-app-secrets.sh, get-github-app-installation-id.sh, list-github-apps.sh, suggest-github-app-name.sh, generate-jwt.sh) and install-mongodb-community-operator.sh. Update scripts/README.md.
• docs/GITHUB_ARC_SETUP.md: now a short retirement note, so the existing link in docs/README.md stays valid. README.md and docs/ARCHITECTURE.md: operator lists updated.

docs/UPGRADE_PLAN.md, docs/OPS_NOTES.md and CHANGELOG.md get updated in the follow-up docs PR for this upgrade wave.

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Note

Medium Risk
Large Terraform deletions change what apply provisions; clusters were already cleaned manually, but anyone relying on those operators or tfvars keys must update config before the next apply.

Overview
Retires GitHub ARC, MongoDB Community, and OpenSearch from the Terraform stack and repo tooling, reflecting the 2026-10-05 live teardown (no CRs were in use on any cluster).

Terraform: Deletes nine null_resource blocks in terraform/main.tf that Helm-installed those operators on nprd/prd/poc apps (including ARC RBAC in managed-cicd). Drops opensearch_operator_version, mongodb_operator_version, and github_arc_controller_version from variables.tf and terraform.tfvars.example—remove them from local terraform.tfvars to avoid undeclared-variable warnings.

Scripts: Removes ARC/GitHub App helpers and install-mongodb-community-operator.sh; scripts/README.md no longer documents ARC or MongoDB install paths.

Docs: README.md and docs/ARCHITECTURE.md now list only CloudNativePG (plus optional Palworld) as automated operators. docs/GITHUB_ARC_SETUP.md is a short retirement note (CI is GitLab runner on nprd-apps); prior guide lives in git history.

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