Investigate what happens when "resources" changes to "_resources"

[emteknetnz]

• In CMS 4, the default resoures folder is "resources"
• In CMS 5, the default resources folder is "_resources"
• The resources folder can be configured via the key "extra.resources-dir" in composer.json
• in CMS 4, if this key is not present, it will now throw a deprecation warning which was added here

There was confusion and discussion about what happens when you change this - #10702 (comment)

Projects that haven't defined "extra.resources-dir" will this effectively changed for them when they upgrade to CMS 5. We don't actually know what happens here, whether there's upgrade friction or not.

If it's a low-friction event, they we could possibly just remove the deprecation notice since the user doesn't need to do anything.

If however it's a high friction event, then we may need to give specific instructions either in the deprecation message or the changelog that it's linked to, or link to some other upgrade documentation

[GuySartorelli]

My opinion is this:

The deprecation notice tells the user they have a deprecated default. It may be that they want to keep that setup in which case action is required. It may be that they want to stop using the deprecated default, in which case action may still be required to validate that they aren't explicitly expecting the old default (e.g. hardcoded references to "resources/my/file.css").

The changelog alerts users to check for this, but having a deprecation warning is also appropriate here. The deprecation warning points to the advice in the changelog.