[Sync to prerelease] Reserve the red announcement banner for prerelease and fix its contrast

[github-actions]

Description

Sync of #2276 to prerelease.

Original PR

Reserve the red announcement banner for prerelease and fix its contrast

The announcement banner colours were mixed up:

Site	Light	Dark
main (before)	#447099	#ff0039
prerelease (before)	#ff0039 (Cosmo $danger)	#ff0039

White on #ff0039 gives 3.95:1. WCAG 1.4.3 needs 4.5:1. The prerelease axe scan flags it on all 334 light/dark pages (run 37517108158). main shows no finding only because it has no announcement now.

This PR reserves red for prerelease, in both modes:

• theme-dark.scss: the main dark banner is #447099, the same as light mode (5.22:1).
• theme-prerelease-docs.scss → theme-prerelease-banner.scss. The new name tells you that the file only styles the banner. The colour is #e60033 (4.76:1).
• _quarto-prerelease-docs.yml: adds the banner file to the dark theme too.

These files are identical on main and prerelease, so the automatic backport should apply cleanly.

Verification:

• I rendered 404.qmd with each profile and checked the compiled CSS. main has #447099 in light and dark. prerelease has #e60033 in light and dark, and it overrides the #447099 rule.
• I ran a full prerelease scan with the same change (in Baseline the code-block findings on website-accessibility #2275, before the split). color-contrast :: #ffffff on #ff0039 cleared (1002 instances), with no regressions.

Conflicts with #2222, which moves these rules into _extensions/quartoorg/scss/.

[cderv]

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