feat: move basic auth for for Azure DevOps to webhooks lib

[robinlieb]

Basic auth for Azure Webhooks was added to the webhooks library in this PR: go-playground/webhooks#191
Which was released with version 6.4.0: https://github.com/go-playground/webhooks/releases/tag/v6.4.0

This change removes the basic auth check for Azure DevOps to remove complexity since this is now included in the underlying library.

Checklist:

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[crenshaw-dev]

Is there any testing we can/should have here?

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 52.85%. Comparing base (da1e95d) to head (d32649c).
Report is 503 commits behind head on master.

Files with missing lines	Patch %	Lines
util/webhook/webhook.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19407      +/-   ##
==========================================
+ Coverage   52.84%   52.85%   +0.01%     
==========================================
  Files         316      316              
  Lines       43656    43636      -20     
==========================================
- Hits        23069    23065       -4     
+ Misses      18032    18020      -12     
+ Partials     2555     2551       -4     

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[robinlieb]

@crenshaw-dev tests for basic auth in Azure DevOps (and secrets for other SCM providers) are already included in the lib. Not sure if it's required in that case. Do you have anything specific in mind?