Update dependency nunit to v4.2.1

Author: renovate[bot]

src/UnitTests/RxTelegram.Bot.UnitTests.csproj

@@ -10,7 +10,7 @@
 
     <ItemGroup>
         <PackageReference Include="NSubstitute" Version="5.1.0" />
-        <PackageReference Include="nunit" Version="4.1.0" />
+        <PackageReference Include="nunit" Version="4.2.1" />
         <PackageReference Include="NUnit3TestAdapter" Version="4.6.0" />
         <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
         <PackageReference Include="System.Reactive" Version="6.0.1" />