Deprecate Support for Travis CI

Summary

This RFC proposes to officially deprecate support for generating a Travis CI config file when creating a new app or addon.

Motivation

Since Travis CI announced the end of its unlimited support for open-source projects, most of the (the entire?) Ember community has switched over to using GitHub Actions instead. This basically leaves the .travis.yml files in the app and addon blueprints unused. Even though the maintenance cost of keeping these files around is pretty low, not having to maintain them would be even better. It would make PRs like this slightly less cumbersome. Also, since almost no one actually uses these files, it becomes harder to know/ensure they are up to date and follow the current best practices.

Transition Path

We should:

• Show a deprecation warning when creating a new app or addon using the --ci-provider=travis option
• Show a deprecation warning when picking the Travis CI option during the interactive new flow
• Add a comment to the .travis.yml files in the app and addon blueprints mentioning that they are deprecated - Adding a comment is the easiest thing to do implementation wise and people who do wish to continue using Travis CI, can simply remove the comment again

How We Teach This

I think we would only need to remove all references to Travis CI from the learning materials.

Drawbacks

Can't think of any at the moment.

Alternatives

Continue supporting Travis CI.

Unresolved questions

None at the moment.