Label color issues with `#84b6eb`

[silverwind]

Description

Labels with background #84b6eb render a white text color, but should actually be black because it's more readable and to match GitHub's algorithm:

Gitea:

GitHub:

Gitea Version

main

Can you reproduce the bug on the Gitea demo site?

Yes

[silverwind]

Related:

#24508
#24586

Issue is present in both frontend and backend color calculations.

cc: @HesterG

[yp05327]

Update GitHub view:

[wxiaoguang]

IIRC it should be right now.