Allow admins to administer personal snippets

What does this MR do?

Updates the PersonalSnippetPolicy to permit admins to administer all personal snippets as if they were the author.

#26380 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content