Squash migrations to 20210602155110

What does this MR do and why?

Squashes migrations to 20210602155110, as well as removes the associated specs and background migration classes.

Also solves #385799 (closed) - I added an exception in the simplecov config script for the \d{14}_init_schema.rb migration to not run afoul of code coverage warnings.

How to set up and validate locally

bundle exec rake db:drop db:create db:migrate

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #370640 (closed)