Make links in gallery captions white

[jffng]

This PR makes a small change to links inside image captions in the gallery block.

Since the caption text is white atop the default black gradient, I think the links also be white. Otherwise, they are unreadable when the default link color is a dark color.

Using TT1:

Before	After

[kjellr]

Just FYI, there's a large Gallery refactor happening in #25940. Not sure if this is necessary or not alongside that work.

[jasmussen]

Kjell makes a good point. There's an open caption question with that ongoing refactor — how will they display? I would hope that they look mostly identical to what exists, that should be the goal, and in that vein, this seems an okay change to land and for the reasons outlined. But definitely something to keep an eye on.

[jffng]

Ah thanks for that context. It appears that this change would create to conflict when that PR lands because of some scss refactoring and the addition of a "deprecated" sass file: https://github.com/WordPress/gutenberg/blob/0550bccc96d2115945da2d39212218a60fcd1baa/packages/block-library/src/gallery/deprecated.scss

I'll wait until that PR lands as it's quite active and revisit this change then... Thanks for the eyes!

[jffng]

Closing as this is well out of date at this point and the gallery refactor looks close to landing.