[upstream] [Style] Support adding class attribute to table captions

Problem/Motivation

When configuring a style for caption elements, it cannot be applied to the table plugin model through the styles drop-down. Incidentally, it also cannot be applied through source editing mode, as it's stripped out, which might be a GHS issue.

See https://github.com/ckeditor/ckeditor5/issues/13777 for the blocking upstream issue.

Steps to reproduce

1. Add caption.test|Caption test as a style option.
2. Observe that the style cannot be applied via the styles dropdown.
3. Add <caption class="test">Bah humbug</caption> via source editing mode, switch out of source editing mode, and back in, observing that the class attribute is gone.

Proposed resolution

Wait for the upstream issue to be resolved; upgrade; profit.

Remaining tasks

TBD

User interface changes

None (hopefully)

API changes

None (hopefully)

Data model changes

None (hopefully)

Release notes snippet