Erlang 24 compatibility

Note to other fellow maintainers: When Erlang or Elixir does a release candidate, test it.

ArchLinux folks having breakages with major updates is normal for them but "Erlang Solutions" pulled this on us…

Deprecations: https://github.com/erlang/otp/blob/master/system/doc/general_info/DEPRECATIONS

• :crypto.hmac → :crypto.mac(:hmac, …) in pleroma (requires OTP 22+)
• web_push_encryption: https://github.com/danhper/elixir-web-push-encryption/pull/27

Closes: #2647 (closed)