Guest users should not be able to delete project snippets

What does this MR do?

Add policy to ProjectSnippets to prevent a guest from deleting their own snippets. Only users that are members of the project with reporter access or greater can delete project snippets they have created.

Screenshots

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

Issue: #31836 (closed)