Deprecate Buffer::count_set_bits

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Buffer::count_set_bits is easy to incorrectly use, as Buffer does not know its accurate length in bits, only in bytes.

This led to #3061

Describe the solution you'd like

I would like to deprecate Buffer::count_set_bits in favour of Buffer::count_set_bits_offset which is less easy to accidentally misuse.

Describe alternatives you've considered

Additional context

[viirya]

Looks good to me. I think mostly we care about count_set_bits_offset, not count_set_bits.

[alamb]

label_issue.py automatically added labels {'arrow'} from #3062