chore: Remove deprecated AddCertificateAuthentication extensions

[Roeland54]

Closes #355

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[Roeland54]

Seams like the api of nuget.org was not accessible from the pipeline when the integration tests where running?

[stijnmoreels]

Seams like the api of nuget.org was not accessible from the pipeline when the integration tests where running?

Had it on other Arcus libraries, too. Was bc the MyGet storage was at it limits 😅 . Should be fixed.

[stijnmoreels]

Great job in removing this, and thx so much for not ruining the code formatting and only removing whitespace. Is greatly appreciated! 🎉
And thanks again for all your PR's on this library! Super enthusiasm.

[stijnmoreels]

/azp run CI - Arcus.WebApi

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