How to change C# Language Version for all of the projects in my solution in one place?

To set a version for all your project at once, you can create a file named Directory.Build.props (case-sensitive on Linux) at the root of your repository. This file contains the list of common properties of your projects:

<Project>
  <PropertyGroup>
    <LangVersion>latest</LangVersion>
    <!--<LangVersion>preview</LangVersion>-->
    <!--<LangVersion>7.3</LangVersion>-->
  </PropertyGroup>
</Project>

https://www.meziantou.net/4-ways-to-enable-the-latest-csharp-features.htm#method-4-using-a-pro