Modernize TagBot.yml

[timholy]

I have started getting daily github notifications:

Your TagBot workflow should be updated to use issue comment triggers instead of cron.
See this Discourse thread for more information: https://discourse.julialang.org/t/ann-required-updates-to-tagbot-yml/49249

This should silence them 🙂. I'm not sure whether the DOCUMENTER_KEY is working, but we'll find out soon I guess.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.18%. Comparing base (8c25bad) to head (3d63f36).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #604   +/-   ##
=======================================
  Coverage   87.18%   87.18%           
=======================================
  Files          28       28           
  Lines        1888     1888           
=======================================
  Hits         1646     1646           
  Misses        242      242           

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