Use `DestroyService` when an admin deletes an SSH Key via the API

What does this MR do?

Contributes to #220127 (closed)

We are not adding AuditEvent creation to Keys::DestroyService now as the refactoring of audit_events table is going on, and we have temporarily halted adding of new audit event types till that is complete.

This change is just a tech-debt fix, so that wherever SSH keys are destroyed across the codebase, Keys::DestroyService is used.

Once we add AuditEvent generation to this DestroyService, audit events will be recorded from all these places - that is the larger plan.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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