Block LFS writes in read-only mode

What does this MR do?

This MR is a result of the discussion !46045 (diffs, comment 441993716).

We want to only allow LFS pulls in FOSS, but on Geo secondaries, we want to allow writes as well, since they proxy those to the primary.

Screenshots (strongly suggested)

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.