New/Set-GitHubRepository: Add Support for 'Allow Update Branch' Option

[X-Guardian]

Description

This PR adds the AllowUpdateBranch parameter to the New-GitHubRepository and Set-GitHubRepository functions:

Issues Fixed

• Fixes #386

References

• https://docs.github.com/en/rest/repos/repos#create-an-organization-repository
• https://docs.github.com/en/rest/repos/repos#update-a-repository

Checklist

• [x] You actually ran the code that you just wrote, especially if you did just "one last quick change".
• [x] Comment-based help added/updated, including examples.
• [x] Static analysis is reporting back clean.
• [x] New/changed code adheres to our coding guidelines.
• [ ] Formatters were created for any new types being added.
• [x] New/changed code continues to support the pipeline.
• [ ] Changes to the manifest file follow the manifest guidance.
• [x] Unit tests were added/updated and are all passing. See testing guidelines. This includes making sure that all pipeline input variations have been covered.
• [ ] Relevant usage examples have been added/updated in USAGE.md.
• [ ] If desired, ensure your name is added to our Contributors list

[X-Guardian]

@HowardWolosky, this PR is ready for review.

[HowardWolosky]

/azp run PowerShellForGitHub-CI

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