Migrate from secrets to credentials

[lomky]

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• What are we trying to do?

Right now we use the (quite old) style secrets file. Let's move over to using the new-style configs/credentials/ files

• What feature or behavior is this required for?

Keeping up with Rails Norms

• How could we solve this issue? (Not knowing is okay!)

After a bit of research, let's follow the per-env key migration route. That best matches our current setup.

This article has a good step by step.

Let's create a development.key, test.key, run the rails commands to set up each of their credential files. All those can be committed to the repo.
Remove the secrets file. Update the .gitignore.
Either @colinxfleming or @lomky will handle generating the production.key and its corresponding credential file before merging, updating the production env var, & releasing it!

• Anything else?

S/o to the work done in #3128 & #3220 for kicking this off!