Follow-up: Add validation to integrations to enforce project, group, or instance level availability

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This issue was created from this suggestion !162383 (comment 2059840234).

Proposal

should we add some validation to Integrations::JiraCloudApp that prohibit setting instance field to true?

Add a new validation to Integration, that will apply to all integrations, to prohibit setting instance property to true based on .available_integration_names. And similarly prohibit setting a group_id or project_id unless the integration is available at those levels.

The change should be behind a feature flag as it has the potential to be disruptive.

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