[chore] Drop .NET 5.0 support, add .NET 9.0 support

[nwithan8]

Description

• Drop .NET 5.0 support for EasyPost library
  • Remove .NET 5.0 from target frameworks for library and test suites
  • Remove .NET 5.0 target framework and binaries from NuGet package build
  • Remove .NET 5.0 from CI
  • Remove .NET 5.0 installation via environment setup scripts
• Add .NET 9.0 support
  • Add .NET 9.0 to target frameworks for library and test suites
    • Use .NET 9.0 for FSharp and VB compatibility check
  • Add .NET 9.0 target framework and binaries to NuGet package build
  • Add .NET 9.0 to CI
  • Add .NET 9.0 installation via environment setup scripts

Testing

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[nwithan8]

Good thing to know: Accidentally confirmed that even when .NET 5.0 is not explicitly listed as a target framework in the library, it did still run in a .NET 5.0 environment successfully. Given that we haven't made any framework-breaking changes, it's likely that our library will continue to work in .NET 5.0 (and probably .NET Core 3.1) even if we don't explicitly support/test it in those versions.