Requirements for v9 release

[gfellerph]

Mind dump of all the things that need to be finished before v9 can go to prod.

• Clean up changesets. During the development of v9, many changesets have been added and not all of them might be relevant anymore - especially bugfixes that were done between the initial creation of a new component and the release. These infos are unnecessary because the component is not used anywhere prior to prod release.
• Rewire release action to main again. Due to automations around the release process, creating netlify sites and the decision to develop v9 on the main branch instead of another, we had to rewire prod publications of the docs to the release/v8 branch to be able to still release fixes. This process needs to be reverted in order to release prod docs from main again.
• Remove bootstrap. All references to bootstrap need to be removed and the dependency needs to be uninstalled. We've started it, we've promised it, we're almost there, we need to finish it. We can no longer upgrade bootstrap without major investment in manual testing, the token system is not compatible with the bootstrap way of handling color schemes.
• Align @swisspost/internet-header versions with the rest of the Design System. During v8, the Internet Header major version was not aligned with the rest of the Design System because it was not necessary to release a major version for this package, which would have had big consequences for all projects using it. With the redesign, a new major version will be released and we can align it with the rest of the Design System. That will be required for the future because there will be a tight dependency on the components package. Fixes for internet-header v1 can be done on the release/v8 branch.
• Spotless migration instructions. This point will decide on whether projects are willing to upgrade or trying to delay because it seems too much. With a clear and easy to follow instructions, we can ease a lot of the pain that comes with major upgrades of this magnitude. Deprecations for styles v9 #1215 is a reference for breaking changes, but not all breaking changes are listed there
• Automigration solution. Come up with a new way to do automigrations, even if it's just search & replace. Having a tool in place will lower the barrier for upgrading.