Simplify gradient color specifications from rgba to rgb in theme.json

[georgestephanis]

Pushed through to Gutenberg via ​https://github.com/WordPress/gutenberg/pull/70008 however that file does not sync to core, so doing a standalone ticket here.

linked gh pr incoming in lieu of a .patch file

Currently, the theme.json that ships in core at wp-includes/theme.json default gradients are inconsistent in color specification -- a number of places it uses rgba(#,#,#,1) even though that is equivalent to just rgb(#,#,#) -- there is no rhyme or reason I can see as to why rgba is used here, so this is to trim a couple bytes by serving them up as rgb instead of rgba.

As there is no transparency here, it may be slightly more efficient to use hex? But I'm fine leaving it as RGB for less code churn in core unless anyone feels strongly here.

More discussion in the Gutenberg repository, but it was merged there and recommended to be made as a core ticket by @t-hamano

@link ​https://github.com/WordPress/gutenberg/pull/70008

[SergeyBiryukov]

In 60234:

Coding Standards: Simplify gradient color specifications in theme.json.

rgba(#,#,#,1) is equivalent to rgb(#,#,#) and can be replaced with the latter for more consistency.

Original PR from Gutenberg repository:

• ​#70008 Simplify color specifications from rgba to rgb

Follow-up to [50959], [55405].

Props georgestephanis, wildworks, juanfra.
Fixes #63381.