Admin list table invalid CSS property

[manikmist09]

since there is opacity so it should be rgba instead of rgb

[SergeyBiryukov]

Introduced in [44973].

[SergeyBiryukov]

Per ​https://developer.mozilla.org/en-US/docs/Web/CSS/color_value:

Note: As of CSS Colors Level 4, rgba() is an alias for rgb(). In browsers that implement the Level 4 standard, they accept the same parameters and behave the same way.

Apparently this is not an error. When testing in Chrome, both rgba() and rgb() work as expected here.

However, I guess it still makes sense to make the change for consistency with the rest of admin styles.

[SergeyBiryukov]

In 46744:

Administration: Switch rgb() CSS property for paused plugin error message in Plugins list table to rgba().

While technically correct as is, the latter is more consistent with other admin styles and causes less confusion.

Props manikmist09.
Fixes #48629.