ci: bump actions/setup-dotnet from 4 to 6 - #1

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

GetAsync calls TryParse but does not explicitly check whether the conversion succeeded. Either use the return value in a conditional statement or verify that the call site expects that the out argument will be set to the default value when the conversion fails. (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1806)

Use 'string.StartsWith(char)' instead of 'string.StartsWith(string)' when you have a string with a single char (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1865)

Type 'ResolverPipeline' owns disposable field(s) '_http' but is not disposable (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1001)