Replaces .disabled with :disabled and [disabled]

[flavianunes]

• Fixes: Replace .disabled with :disabled and/or [disabled] #791

As @shawnbot suggested, I replaced .disabled with both :disabled and [disabled].

/cc @primer/ds-core

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[simurai]

Changes look good. 👍 Thanks @flavianunes. 🙇

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@shawnbot Since there are probably a lot of places on github/github that we would need to update, should we test this first with an RC from this PR and only once everything is ok, merge this PR into 14.0.0 or 15.0.0?

And by "this PR", I mean merge the changes into a new branch/PR on this repo so that the publish script works.

[shawnbot]

Thank you for this, @flavianunes! @simurai, I think we should hold off on releasing this until the next major release (15.0.0). We've got a lot that needs to change already, and with custom properties on the horizon we've got plenty to queue up already.

[simurai]

An alternative to this PR could be using [aria-disabled=true], see #791 (comment)

And because currently .disabled gets used a lot, as a first step we could only add [aria-disabled=true] and update the docs. Similar to #982. And later remove .disabled once we cleaned up all the uses on dotcom.