Warn customers that Repository Storages config can't be updated via application settings API in 13.1 and 13.2

Issue #291 (closed), follow-up on !224 (merged)

The MR adds a warning for customers on 13.1.0 < version < 13.2.2 if they try to generate vertical data via the Generator. The breaking change with repository_storages_weighted gitlab-org/gitlab#227408 (closed) was introduced in %13.1. While the fix will land in 13.2.2 per gitlab-org/gitlab!36376 (comment 387582864) (🎉 ), we have a gap between the bug and the fix. This means that if a customer tries to generate vertical data having multiple Gitaly nodes - large projects won't be spread evenly across all storages and this will lead to incorrect test results.

We need to guide users on how to import large projects manually for each Gitaly node.