Global Styles: Make strings translatable

[pedro-mendonca]

Make 'Solid' and 'Gradient' labels translatable in the global styles color palette settings.

What?

Add missing gettext to translation strings

Why?

To allow translation of labels

How?

Wrap with __()

Testing Instructions

1. Open a post or page in a translated WordPress.
2. Go to Styles > Colors
3. Open color schene
4. See the 'Solid' and 'Grandient' untranslated

Screenshots or screencast

Fixes #60142

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: pedro-mendonca <pedromendonca@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[swissspidy]

Probably should use _x() in the future, but that needs to be done holistically across the project

[pedro-mendonca]

I agree, such a small string without a context can lead to confusion.

Still, these already exist in the translatiaon tables without context, it's better to do it for all at once.