Add : `void` return type to WebformSubmissionDevelGenerate::generateSubmissions()

Also upload a patch compatible with 6.3.0-beta2.

Thanks for the patch.

Why is this change required? Please update the issue summary.

Tests are not passing.

This MR is an API cleanup, which I think is okay.

I see it is a protected method and its return value is not used. In theory someone could sub-class this and use the return value. This could have a change record. I have updated the issue title to make it more clear what is happening, so perhaps the release notes are sufficient, rather than using a change record.