Upgrade Elixir and Erlang/OTP to 1.14 and 24.3.4, respectively

[Annopaolo]

VerneMQ still does not support OTP 25 (see https://github.com/vernemq/vernemq#building-vernemq). Also, remove deprecated use Mix.Config and use the Config module instead.

[codecov[bot]]

Codecov Report

Merging #69 (63f869e) into release-1.1 (a490959) will decrease coverage by 0.84%. The diff coverage is n/a.

:exclamation: Current head 63f869e differs from pull request most recent head 43b22a9. Consider uploading reports for the commit 43b22a9 to get more accurate results

@@               Coverage Diff               @@
##           release-1.1      #69      +/-   ##
===============================================
- Coverage        78.28%   77.44%   -0.84%     
===============================================
  Files               11       11              
  Lines              221      235      +14     
===============================================
+ Hits               173      182       +9     
- Misses              48       53       +5     

see 5 files with indirect coverage changes

[rbino]

Nice

[Annopaolo]

Superseded by #73 (not nice).