`karapace.protobuf.encoding_variants.write_varint` is broken

### What happened?

Another bug found while adding type-hints. `BytesIO.write()` does not accept an int, which is always the result of subscripting a `bytearray`. There does not seem to be any unit test coverage here.

https://github.com/aiven/karapace/blob/0d0ede8edbba65cc944bb5fbc5df28fdfcdd3bd3/karapace/protobuf/encoding_variants.py#L48

```pycon
>>> from karapace.protobuf.encoding_variants import *
>>> b = BytesIO()
>>> write_varint(b, 123)
Traceback (most recent call last):
File "", line 1, in
File "/Users/anton/aiven/karapace/karapace/protobuf/encoding_variants.py", line 65, in write_varint
bio.write(bytearray(to_write)[0])
TypeError: a bytes-like object is required, not 'int'
```

Not entirely clear what to do about this as it indicates that quite a lot of code is unused, i.e. if this breakage isn't affecting functionality.