Handle max_import_size in GPT Data Generator for 13.4

As detailed in gitlab-org/gitlab#251106 (closed) max import size was recently enabled in GitLab by default and unfortunately with the current timing it will make it into the 13.4 release. This means we need to adjust Generator now to handle it.

Approach should probably be the same as repository_storages where we just disable it (for versions where it is an option, apparently 13.1 and up) for the duration and reset it back to where it was beforehand after.