Allow all snippets to be accessed by API

What does this MR do?

Previously, you could only access snippets via /api/v4/snippets/:id[/raw] if you had authored them. The documentation doesn't state that this is the case, and it's quite surprising.

More surprising is that all these API endpoints allow access to project, as well as personal, snippets - I haven't changed that, but it's perhaps something we should change in the future.

What are the relevant issue numbers?

Closes #41888 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer