Update install.md to reference NET 8 (#1744)

I just tried to run the `dotnet tool install -g git-credential-manager`
command and it failed because I did not have .NET 8 installed. So
updating the docs.

docs/install.md

@@ -210,7 +210,7 @@ the preferred install method for Linux because you can use it to install on any
 distribution][dotnet-supported-distributions]. You
 can also use this method on macOS if you so choose.
 
-**Note:** Make sure you have installed [version 7.0 of the .NET
+**Note:** Make sure you have installed [version 8.0 of the .NET
 SDK][dotnet-install] before attempting to run the following `dotnet tool`
 commands. After installing, you will also need to follow the output instructions
 to add the tools directory to your `PATH`.