Rename the span_frames_min_duration property

The `span_frames_min_duration` seems to be used to decide whether or not to collect stack traces for a span.
The problem is that this information only becomes clear after reading the full documentation, as the name of the property does not make this easy to guess.
It would be better if the name of the property was clear enough for the user to know what it is used for.
Maybe `stack_trace_span_min_duration`, for example.

## Spec issue
- [x] https://github.com/elastic/apm/issues/528

## Agent issues
- [x] https://github.com/elastic/apm-agent-java/issues/2206
- [x] https://github.com/elastic/apm-agent-dotnet/issues/1529
- [x] https://github.com/elastic/apm-agent-go/issues/1142
- [x] https://github.com/elastic/apm-agent-php/issues/568
- [ ] https://github.com/elastic/apm-agent-ruby/issues/1174
- [x] https://github.com/elastic/apm-agent-nodejs/issues/2381
- [x] https://github.com/elastic/apm-agent-python/issues/1369