Label enforcer workflow: make accessibility a focus instead of a type

[priethor]

What?

Removes the limitation of having the Accessibility label along with types of labels like Bug or Enhancement.

At the same time, if this change is approved, a PR will need to have a type-of-task label associated with it. The reasoning behind this is multiple, as not having clear rules for labeling makes it more complicated to contribute and to report and do project management on the project.

Why?

As mentioned by @afercia here.

Regarding the accessibility label, quoting from #54724 (comment)

I think I already pointed out in some other discussion or issue that Accessibility is typically something that spans across different areas of a project. As such, we should be able to add the Accessibility label everywhere. WhethEr it's a Type or other, I don't have strong preferences.

Accessibility in Core Trac used to be a component and was changed to Focus right for that reason. The re=organizazion of the Trac focuses dates early 2014. For more details and the reasoning about Accessibility as focus, see https://make.wordpress.org/core/2014/01/24/trac-focuses/

How?

By removing the label from the list of type labels.

If this PR is approved, I will follow up with renaming the label to remove the [Type] prefix.

[Mamaduka]

Thank you, @priethor!

[alexstine]

I'm good with this approach.