Refactor vulnerability header

What does this MR do and why?

This refactors to the vulnerability header component. Instead of passing all the individual conditions. We abstract that into a single condition and passed that down as a prop:

- :explain-with-ai-ability="glAbilities.explainVulnerabilityWithAi"
- :ai-explanation-available="vulnerability.aiExplanationAvailable"
+ :show-explain-with-ai="canExplainWithAi"

This also makes it consistent with the rest of the actions:

 :show-download-patch="canDownloadPatch"
 :show-create-merge-request="canCreateMergeRequest"
+:show-explain-with-ai="canExplainWithAi"
+:show-resolve-with-ai="canResolveWithAi"

MR acceptance checklist

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Related to #490302 (closed)