Update generate_perf_testdata script to create many files on master

What does this MR do?

This MR addresses a couple of issues discovered during run against a GitHost migration source instance.

It updates script to create many files on master and moves create_many_issues out of threaded run.

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content