Set webhook content type (#15952)

[sgross-emlix]

Description

Set correct content type for webhook's json payload.

Fixes #15952

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

See related issue's reproduction steps

Result is now : Your General Webhook Integration works!
And the gitlab pipeline is actually triggered

Checklist:

• I have read the Contributing documentation available here: https://snipe-it.readme.io/docs/contributing-overview
• I have formatted this PR according to the project guidelines: https://snipe-it.readme.io/docs/contributing-overview#pull-request-guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

[welcome]

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[what-the-diff]

PR Summary

• Improved HTTP requests handling
  By incorporating the use GuzzleHttp\Psr7\Request; line in our code, our practices for handling HTTP requests have been streamlined and made more efficient.

• Updated method within 'testWebhook' function
  We've altered the post method inside our testWebhook function so that it now uses the send method together with a Request object. This update improves the overall performance of the function, giving us a faster and more reliable tool for our testing needs.

• Corrected content-type header mistake
  In the functions googleWebhookTest and msTeamTestWebhook, we've corrected an error in the content-type header. It used 'applications/json' instead of 'application/json'. This correction ensures better compliance with the accepted standards and boosts the interoperability with other services adopting the same data format.

• Enhanced Readability with the introduction of Headers Variable
  To ensure our code is easier to read and follow, we've introduced a headers variable in the testWebhook function. A step that makes our code cleaner and supports the speedy identification of any potential issues in the future.

[sgross-emlix]

On second thought one might want to get rid of the Guzzle Client and use the Facades/Http class for everything. This might reduce code redundancy. But my php skill is too low and was merely enough for this hotfix in our instance.

[Godmartinz]

@sgross-emlix could you retarget this PR to develop? thanks

[snipe]

I've fixed that typo on develop and master. Thanks for the heads up!