text: Title: Type conversion: Remove support for deprecated `ByteString`
  Body: [collections.abc.ByteString](https://docs.python.org/3/library/collections.abc.html#collections.abc.ByteString) that Robot's argument conversion supports has been deprecated in Python 3.12 and will be removed in Python 3.14. This basically makes the current Robot Framework code incompatible with Python 3.14 that's released already in 2025, and also means that there are deprecation warnings with Python 3.12 already now.
  
  `ByteString` conversion isn't that useful and I believe the best way to handle these problems is to simply not support it. This is how conversion currently works if you use `arg: ByteString`:
  
  1. If the given argument is an instance `bytes` or `bytearray` (i.e. is already an instance of `ByteString`), it is passed to the keyword as-is.
  2. Otherwise, it is converted to `bytes` the same way as if the keyword would have used `arg: bytes`.
  
  I don't consider these semantics too useful. I don't see how often you'd like to have a keyword that gets either `bytes` or `bytearray`, when you could use `arg: bytes` or `arg: bytearray` to get one of them. More importantly, if you have such a need, using `arg: bytes | bytearray` works exactly the same way and makes then intention more clear.
  
  Removing the support for `ByteString` conversion is obviously a backwards incompatible change, but I doubt there are many users for it, they can easily use `bytes | bytearray` instead, and they anyway need to do that relatively soon when `ByteString` will be gone. I wish we would have done this earlier in RF 7.0 development cycle, but believe doing it still now before the release candidate is fine.
  
  