Use GitHub Actions for Heroku deployment

[ml-evs]

Due to ongoing security issues with Heroku, they have disabled the direct GitHub integration.

This PR uses GitHub actions to deploy to Heroku via the https://github.com/AkhileshNS/heroku-deploy Action (20k+ users, seems fine).

The required credentials have been added as GH actions secrets (using my Heroku account API key).

I tested this on my fork and it works fine, this will only run after it is merged to master so I will double-check afterwards.

We can then disable Heroku automated deployment, remove the config vars from Heroku, and upgrade the heroku stack to v20.

Closes #1158.

[codecov]

Codecov Report

Merging #1165 (b687748) into master (255a402) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1165   +/-   ##
=======================================
  Coverage   92.32%   92.32%           
=======================================
  Files          68       68           
  Lines        3894     3894           
=======================================
  Hits         3595     3595           
  Misses        299      299           

Flag	Coverage Δ
project	92.32% <ø> (ø)
validator	92.32% <ø> (ø)

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[ml-evs]

Going to self-merge as this can only really be tested once its in!

[ml-evs]

Failed due to missing credentials, need to rerun from master.