refactor(ui): rename surface tokens to neutral background tokens

Replace --bui-surface-1..5 with --bui-bg-neutral-1..5 for both light
and dark themes. --bui-bg-neutral-1 replaces the deprecated --bui-bg-app,
and --bui-bg-neutral-2..5 extend the scale. The old bare --bui-bg-neutral-1..4
entries are removed from the deprecated section since their names are now
reused; the hover/pressed/disabled variants remain deprecated.

Updates colors.stories.tsx, the ESLint rule and tests, and all migration
documentation and the PR description accordingly.

Signed-off-by: Charles de Dreuille <charles.dedreuille@gmail.com>

packages/eslint-plugin/rules/no-deprecated-bui-tokens.js

@@ -23,19 +23,15 @@ const DEPRECATED_TOKENS = [
   '--bui-bg-solid-pressed',
   '--bui-bg-solid-disabled',
   '--bui-bg-app',
-  '--bui-bg-neutral-1',
   '--bui-bg-neutral-1-hover',
   '--bui-bg-neutral-1-pressed',
   '--bui-bg-neutral-1-disabled',
-  '--bui-bg-neutral-2',
   '--bui-bg-neutral-2-hover',
   '--bui-bg-neutral-2-pressed',
   '--bui-bg-neutral-2-disabled',
-  '--bui-bg-neutral-3',
   '--bui-bg-neutral-3-hover',
   '--bui-bg-neutral-3-pressed',
   '--bui-bg-neutral-3-disabled',
-  '--bui-bg-neutral-4',
   '--bui-bg-neutral-4-hover',
   '--bui-bg-neutral-4-pressed',
   '--bui-bg-neutral-4-disabled',