OTP-21 support

OTP-21 support

- conditionally drop metrics related to error_logger queue length
  when the process is not running
- deprecate recon:files/0 since the file implementation in OTP-21
  works with NIFs and no longer allows listing descriptors
- updating tests to work with the new constraints -- apparently polling
  a process increases its reduction count now as well
- move the CI to rebar3
- have the docsite script updated to deal with the removal of non-SMP
  erlang
- bump minor version
- make support of R15 and R16 a best-effort thing