calendar: Adjust colors to improve contrast, emphasis

The selected red for "unstable" has very poor contrast in dark mode, and the use of thin font for stable and old-stable releases causes them to appear de-emphasised compared to the eol release, which uses a normal font.

Borrow some colours from libadwaita for "warning" and "danger" which have light and dark mode variants, and use "warning" on unstable releases, and "danger" on eol releases.

De-emphasize old-stable releases slightly by making the font color semi-transparent.

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