Add Erlang 24 support

[dumbbell]

There are three parts in this pull request:

1. An update to the GitHub workflow to test against Erlang 23 and 24, thanks to @pjk25.
2. Various changes to the testsuites to make them work on Erlang 24.
3. A change to the message encoding to fix the library on Erlang 24.

Without the latest point, RabbitMQ master branch fails to notify systemd with the {error, {invalid, {iov, {element_not_binary, 1}}}} and the start command hangs.

[codecov[bot]]

Codecov Report

Merging #29 (45cc341) into master (c5b3a5b) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #29   +/-   ##
=======================================
  Coverage   91.41%   91.41%           
=======================================
  Files           8        8           
  Lines         326      326           
=======================================
  Hits          298      298           
  Misses         28       28           

Impacted Files	Coverage Δ
src/systemd_protocol.erl	100.00% <100.00%> (ø)

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[michaelklishin]

@hauleth sorry to mention you personally but this issue is kind of a blocker for team RabbitMQ's pipelines on Erlang 24. Would it be possible to take a look at this PR soon? We are happy to contribute what we need (or fix issues we run into) and would very much like to not use a fork :)

[hauleth]

Looking into it right now.

[hauleth]

I will publish fixed version later today.

[michaelklishin]

Thank you!

[hauleth]

@michaelklishin published as 0.6.1

[hauleth]

💚 💙 💜 💛 ❤️