Replaced by `#lease_connection` to better reflect what it does.

`ActiveRecord::Base#connection` is deprecated in the same way
but without a removal timeline nor a deprecation warning.

Inside the Active Record test suite, we do remove `Base.connection`
to ensure it's not used internally.

Some callsites have been converted to use `with_connection`,
some other have been more simply migrated to `lease_connection`
and will serve as a list of callsites to convert for
https://github.com/rails/rails/pull/50793