Upgrade to Rebar 3.20 about riak

[martinsumner]

A note on OTP support. Riak 3.0.15+ is currently OTP 22 only. The intention has been for Riak 3.2.1 to be OTP 22 and OTP 25+ (OTP 25 is proven, there's been some initial work on OTP 26).

I have vague memories of past discussions on Slack about whether there should always be an option when upgrading Riak versions to do so without needing an OTP uplift - which I think was the motivation in ensuring backwards compatibility to OTP 22 in 3.2.n. However, I think we need to be pragmatic. There are real benefits of aligning Riak with the community, as well as people willing to help to achieve this. rebar3 has a clear policy that would at the very least constrain supporting both OTP 22 and OTP 26. There is also an ongoing release tax with testing Riak across multiple OTP versions.

So I think just as support was (accidentally) dropped for OTP 20 in 3.0.10, it should be OK to drop support for OTP 22 in Riak 3.2.1 or 3.2.2. So, for example, 3.0.10+ would be OTP 22 only, 3.2.1 would be OTP 25 only and 3.2.2 may be OTP 26 only.

@nsaadouni @hmmr - does this seem OK with you? I think we have been through this loop before, and I may be guilty of forgetting some potential issues.