Fix Gnus logo color customization

Fix Gnus logo color customization

Fix a bug where customizing `gnus-logo-color-style' from use-package's
:custom keyword did not set `gnus-logo-colors' before Gnus is launched.

This patch does the following:
- Implements a :set keyword on `gnus-logo-color-style' which will
  correctly set `gnus-logo-colors'
- Exposes `gnus-logo-colors' using `defcustom' for more fine-grained
  customization
- Uses :set-after on `gnus-logo-colors' so that it is set after
  `gnus-logo-color-style'

* lisp/gnus/gnus.el (gnus-logo-colors): Convert to defcustom.
(gnus-logo-color-style): Add :set property.  (Bug#74554)

(cherry picked from commit 77d0078a6c50c95b63ac7c3739ee325264deff34)