Fix plugin deactivation link color

[felixarntz]

Summary

The plugin deactivation link is currently bright red (via a simple color: red hard-coded in a style attribute). This is not a good color choice, as it is very bright and doesn't follow the WordPress admin color scheme. It should instead use #b32d2e which is used for other deactivation links on a white background (e.g. in the plugins list table).

This PR fixes that, and as part of it also avoids the style attribute and instead colocates the style with the other styles for the admin page.

This PR uses no milestone to not include it in changelogs, since for end users this bug is not relevant - it only will be if we don't fix it now :)

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.