Bump MimeKit from 4.8.0 to 4.9.0, and net9 (#33)

* Bump Nerdbank.GitVersioning from 3.6.146 to 3.7.112

Bumps [Nerdbank.GitVersioning](https://github.com/dotnet/Nerdbank.GitVersioning) from 3.6.146 to 3.7.112.
- [Release notes](https://github.com/dotnet/Nerdbank.GitVersioning/releases)
- [Commits](https://github.com/dotnet/Nerdbank.GitVersioning/compare/v3.6.146...v3.7.112)

---
updated-dependencies:
- dependency-name: Nerdbank.GitVersioning
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

* Bump MimeKit from 4.8.0 to 4.9.0

Bumps [MimeKit](https://github.com/jstedfast/MimeKit) from 4.8.0 to 4.9.0.
- [Changelog](https://github.com/jstedfast/MimeKit/blob/master/ReleaseNotes.md)
- [Commits](https://github.com/jstedfast/MimeKit/compare/4.8.0...4.9.0)

---
updated-dependencies:
- dependency-name: MimeKit
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

* Bump NUnit from 4.2.2 to 4.3.0

Bumps [NUnit](https://github.com/nunit/nunit) from 4.2.2 to 4.3.0.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/main/CHANGES.md)
- [Commits](https://github.com/nunit/nunit/compare/4.2.2...4.3.0)

---
updated-dependencies:
- dependency-name: NUnit
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

* Update to net9

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Smaug123 <3138005+Smaug123@users.noreply.github.com>

flake.nix

@@ -14,8 +14,8 @@
     flake-utils.lib.eachDefaultSystem (system: let
       pkgs = nixpkgs.legacyPackages.${system};
       pname = "Dmarc-dotnet";
-      dotnet-sdk = pkgs.dotnet-sdk_8;
-      dotnet-runtime = pkgs.dotnetCorePackages.runtime_8_0;
+      dotnet-sdk = pkgs.dotnetCorePackages.sdk_9_0;
+      dotnet-runtime = pkgs.dotnetCorePackages.runtime_9_0;
       version = "0.1";
       dotnetTool = dllOverride: toolName: toolVersion: hash:
         pkgs.stdenvNoCC.mkDerivation rec {