Remove :shared from a valid feature categories for tests

What does this MR do and why?

Removes :shared as a valid feature category for tests, as it causes ownership ambiguity that requires unnecessary human intervention in the automated quarantine process.

Part of Epic gitlab-org/quality&195. When tests are quarantined automatically, those with feature_category: :shared cannot be assigned to a specific team, resulting in:

• Quarantine MRs sitting unreviewed for months
• Manual triage needed to determine ownership
• Broken automation workflow

This change enforces clear team ownership for all new tests, enabling fully automated quarantine MR assignment.

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