func azure functionapp -h

Under the publish command...

--dotnet-version              Only applies to dotnet-isolated applications. Specifies the .NET version for the function app. For example, set to '5.0' when publishing a .NET 5.0 app.

.NET 5 is no longer supported. This example should be updated to .NET 6, which is the most recent GA LTS version.

Technically, it also shouldn't be "... publishing a .NET 6.0 app." but rather "... publishing a .NET 6 app.". Only the major version should be specified when using the brand term. The setting value for the parameter is fine.