[ENHANCEMENT] Deprecate outputPaths build option

[bertdeblock]

Closes #9478.

[bertdeblock]

Draft, as I don't think we can land this before v5.1.0.

[locks]

@bertdeblock I think it's time 😅 Might need to update the version values.

[NullVoxPopuli]

what would they be? 5.2 or 5.3?

5.2 is still beta, so.. 5.2?
(or does beta have its own branch, so we'd have to do 5.3?)

[bertdeblock]

Updated. Beta has its own branch, so master should be 5.3.0 at the moment. Made this MR a while ago, so best to check if we actually can / want to land this deprecation at this time.

[boris-petrov]

I can't seem to find a migration path anywhere. What are we supposed to change outputPaths with?

[bertdeblock]

It was deprecated without a replacement if I remember correctly:
https://deprecations.emberjs.com/ember-cli/v5.x#toc_ember-cli-outputPaths-build-option

[boris-petrov]

I am using it to output different CSS files for different themes. So I'll have to check how to do that with webpack I suppose?