apply rdbms support policy

[christinaausley]

Description

Closes #7317. This PR applies Increment 1 of the RDBMS Support Policy rollout across the documentation. The goal is to ensure that database-related pages consistently reference the new RDBMS Support Policy and use terminology that matches the updated Storage Architecture.

What was updated

• Added links to the RDBMS Support Policy in pages that reference supported databases or database version compatibility.
• Removed outdated or incorrect statements about database support, availability, or limitations.
• Updated terminology to align with the new Primary and Secondary storage model.
• Cleaned up older references that implied Elasticsearch was the only supported secondary storage backend.
• Updated several pages to reflect current storage architecture behavior.

Notes

Because database references appear across many areas of the documentation set, it can be difficult to catch every outdated phrase or legacy mention in a single pass. As we continue working in related areas and expanding RDBMS support in future increments, we should keep an eye out for remaining terminology inconsistencies and update them as we encounter them.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support. (add bug or support label)
• This is already available but undocumented and should be released within a week. (add available & undocumented label)
• This is on a specific schedule and the assignee will coordinate a release with the Documentation team. (create draft PR and/or add hold label)
• This is part of a scheduled alpha or minor. (add alpha or minor label)
• There is no urgency with this change (add low prio label)

PR Checklist

• The commit history of this PR is cleaned up, using {type}(scope): {description} commit message(s)

• My changes are for an upcoming minor release and are in the /docs directory (version 8.9).
• My changes are for an already released minor and are in a /versioned_docs directory.

• I added a DRI, team, or delegate as a reviewer for technical accuracy and grammar/style:
  • Engineering team review
  • Technical writer review via @camunda/tech-writers unless working with an embedded writer.

[github-actions]

👋 🤖 🤔 Hello, @christinaausley! Did you make your changes in all the right places?

These files were changed only in docs/. You might want to duplicate these changes in versioned_docs/version-8.8/.

• docs/self-managed/components/management-identity/miscellaneous/configuration-variables.md
• docs/self-managed/components/orchestration-cluster/core-settings/concepts/schema-and-migration.md
• docs/self-managed/concepts/secondary-storage/configuring-secondary-storage.md
• docs/self-managed/concepts/secondary-storage/index.md
• docs/self-managed/deployment/helm/configure/database/using-existing-postgres.md
• docs/self-managed/deployment/helm/configure/registry-and-images/air-gapped-installation.md
• docs/self-managed/deployment/manual/cloud-providers/amazon/aws-ec2.md
• docs/self-managed/quickstart/developer-quickstart/c8run.md

You may have done this intentionally, but we wanted to point it out in case you didn't. You can read more about the versioning within our docs in our documentation guidelines.

[christinaausley]

@volodymyr-melnykc I've added you for review with this and #7434, feel free to delegate if you'd like 👍

[christinaausley]

Ok comments are resolved, ready for final review 👍