Add `travel_back` to remove stubs from `travel` and `travel_to`
Remove automatic removal of Date/Time stubs after each test case

This behavior is only work out-of-box with minitest and also add a
downside to run after each test case, even if we don't used the travel
or travel_to methods
Improve documentation [ci skip]
Merge pull request #13884 from rafaelfranca/rm-travel-back

Add `travel_back` to remove stubs from `travel` and `travel_to` and remove auto-rollback after each test case