Consolidating migrations about maintenance_tasks

[etiennebarrie]

Rails engines provide a $engine_name:install that can be used on the first installation and any upgrade. It's idempotent for the migration files but isn't super smart, it just checks whether the same migration exist based on its class name.

We even have an explanation about this in the README, for people who roll up their migrations and then have our migrations re-appear: https://github.com/Shopify/maintenance_tasks/blob/eeee3f4ebfdeea048c2d3bb9897ca29c2cd93187/README.md#upgrading

While we could provide a special task for first installs (with a rolled up migration) and keep the current migrations for everyone else that need the "diff" migrations, ultimately this should be built in Rails so it can benefit every engine, and not just this one.

[Schwad]

Use bundler to check for and upgrade to newer versions. After installing a new version, re-run the install command:

Okay, that makes sense, so that folks can upgrade in a clear way. Thanks for the explainer