* Parallel migration (#18815)
* flagParallel sanity check
* Attempt to use ErrGroups
* Updated docs
* Allow 'start' and 'max-parallel' together
* parallel flag renamed to max-parallel
* tests for start + parallel
* Removed permit pool
* Updated docs to make it clearer that a high setting might not be honored based on storage backend setting
* System dependent max int size
* Default max-parallel 1 => 10
* Test folder/paths updated
Co-authored-by: Tomasz Pawelczak <10206601+gites@users.noreply.github.com>
Co-authored-by: Mike Palmiotto <mike.palmiotto@hashicorp.com>
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