Upgrade to Stylelint v15

[keichinger]

This change:

• requires at least Stylelint v15
• removes all deprecated rules (see https://stylelint.io/migration-guide/to-15#deprecated-stylistic-rules)
• enables the new rule declaration-property-value-no-unknown: true (see https://stylelint.io/migration-guide/to-15#added-declaration-property-value-no-unknown-rule)

This prevents stylelint from screaming at us whenever we're doing a pnpm update in any of our projects related to the deprecated rules. However, as Stylelint itself writes in their migration guide: The rules were still working fine, they'll just be removed in v16.

I decided to remove all deprecated rules alltogether since the error output looks like this before the actual stylelint-related issues are actually listed, which is significantly way more noise than I prefer:

WDYT @apfelbox @21christiansc

[apfelbox]

Would be great to have a fixer in place, so that we keep the code style lints. What are the options there?

[keichinger]

I guess we'd need to evaluate something like stylelint-prettier (see https://stackoverflow.com/a/54757106/2690438) and see if it's sufficient.