[SM-244] Resolve dotnet warnings

[Hinton]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [x] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Resolve all of the simple dotnet warnings.

Code changes

• src/Api/Models/Request/Accounts/KdfRequestModel.cs: Hides method.
• src/Api/Models/Request/TwoFactorRequestModels.cs: Hides method.
• src/Core/Tokens/DataProtectorTokenFactory.cs: Unknown generic T
• src/Core/Utilities/CoreHelpers.cs: Unused variable.
• src/Core/Utilities/StrictEmailAddressAttribute.cs: Not a valid target for docblock
• test/Api.IntegrationTest/Controllers/AccountsControllerTests.cs: Potential null (safe since it's a unit test)
• test/Billing.Test/Controllers/FreshdeskControllerTests.cs: Hides method.
• test/Infrastructure.EFIntegration.Test/Repositories/CipherRepositoryTests.cs: Async without await

Before you submit

• Please check for formatting errors (dotnet format --verify-no-changes) (required)
• If making database changes - make sure you also update Entity Framework queries and/or migrations
• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team

[Hinton]

QA asked for this to be merged prior to testing.