fix(logging): Replace log-level parameter by GATUS_LOG_LEVEL env var

[TwiN]

Summary

Replace log-level parameter by GATUS_LOG_LEVEL env var

Tweaks implementation of logging levels in #872

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 38.41060% with 93 lines in your changes missing coverage. Please review.

Project coverage is 77.42%. Comparing base (8060a77) to head (dc2e1ec).
Report is 106 commits behind head on master.

Files with missing lines	Patch %	Lines
main.go	0.00%	32 Missing ⚠️
storage/store/sql/sql.go	54.83%	14 Missing ⚠️
client/config.go	18.18%	9 Missing ⚠️
api/endpoint_status.go	0.00%	7 Missing ⚠️
config/config.go	73.91%	5 Missing and 1 partial ⚠️
controller/controller.go	0.00%	4 Missing ⚠️
watchdog/alerting.go	73.33%	4 Missing ⚠️
client/client.go	0.00%	3 Missing ⚠️
alerting/provider/pagerduty/pagerduty.go	0.00%	2 Missing ⚠️
api/spa.go	0.00%	2 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #895      +/-   ##
==========================================
- Coverage   77.55%   77.42%   -0.14%     
==========================================
  Files          72       72              
  Lines        5788     5784       -4     
==========================================
- Hits         4489     4478      -11     
- Misses       1083     1089       +6     
- Partials      216      217       +1     

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