Remove legacy utility classes

[oliverschuerch]

Tasks

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1. Remove the file https://github.com/swisspost/design-system/blob/main/packages/styles/src/utilities/temp/_legacy.scss
2. Mention every class in the migration guide 8 to 9 and explain what else can be used (if there are existing replacer classes, e.g. .bold => .fw-bold, ...)
3. If one of these classes is documented in the v8 docs, add a deprecation message beside it