[6.x] Fix inconsistent dark mode text colours

[jaygeorge]

Description of the Problem

There are some inconsistent dark mode colors for alerts. This is evident when you explore rich content, as reported in #14829

https://ui.statamic.dev/?path=/docs/components-alert--docs&globals=theme:dark

What this PR Does

• Fixes inconsistent dark mode text colours in the Alert component when using native HTML elements (h1–h6, p, code) versus the Heading and Description components.
  • Native elements inherited the alert's variant text colour, while Heading, Description, and code had their own defaults or per-variant overrides — often a shade lighter (e.g. success body emerald-300 vs code emerald-200, warning headings amber-300 vs native headings amber-200).
• All rich content now inherits the alert's variant text colour via text-inherit!. Per-variant overrides for heading, description, and code text colours are removed; variant rules only set backgrounds for code elements.
• Closes Alert component has slightly different text colors in dark mode #14829

How to Reproduce

1. Go to https://ui.statamic.dev/?path=/docs/components-alert--docs&globals=theme:dark