Don't require dompdf/dompdf

[PHLAK]

We use spatie/browsershot for our app's interal PDF rendering and with the introduction of #1304 we plan to switch Cashier over to use browsershot as well (instead of dompdf/dompdf). However, since dompdf/dompdf is still a required package it will still be installed in all environments, even when unused. It would be nice if dompdf/dompdf were no longer a required package to eliminate an unnecessary dependency. However I know simply moving it to suggest isn't straight forward and thus opened this issue to discuss if you think this would be possible and how to go about making that change. If this is possible/desirable I am willing to work on a PR.

[driesvints]

I agree. Attempted a PR here for the next major release since it's a breaking change: #1312

[arborrow]

I commented on #1084 but figured this might be a good place to mention as well. Is there a sense of when 14 might be pushed out? Given CVE-2022-0085 it might be helpful to get the unused dompdf/dompdf package off people's systems.

[driesvints]

Please see #1393