Swap FluentAssertions with xUnit assertions

[ArinGhazarian]

Description

We were using FluentAssertions as our library of choice for almost all of our unit tests. Unfortunately for the latest version 8, they are switching their license from Apache 2 to a paid for commercial use so we will not be able to upgrade to the latest version (>= 8)! For now we can keep staying on < 8 but eventually we should probably move away from using it and switch to a different library like shoudly or we can use xUnit's built in assertions.

With xUnit being a part of the .NET Foundation there is fairly less risk of any dramatic OSS license changes, I think it would be better to just switch to xUnit's built assertions which has a very readable and straightforward. I started with changing the MigrateRepoCommandHandlerTests.cs for gei and realized that it was very fairly easy to switch but given our sheer number of unit tests it is going to take some time to completely switch though.

• [ ] Did you write/update appropriate tests
• [ ] Release notes updated (if appropriate)
• [ ] Appropriate logging output
• [ ] Issue linked
• [ ] Docs updated (or issue created)
• [ ] New package licenses are added to ThirdPartyNotices.txt (if applicable)

[github-actions[bot]]

Unit Test Results

858 tests   858 ✅  20s ⏱️   1 suites    0 💤   1 files      0 ❌

Results for commit 6c8b7ab7.

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
ado2gh	84%	78%	631	✔
Octoshift	87%	76%	1345	✔
bbs2gh	82%	76%	669	✔
gei	81%	72%	592	✔
Summary	84% (7176 / 8509)	76% (1656 / 2190)	3237	✔