コマンドラインでdotnet efコマンドをアップデートする.

The EF Core tools version '3.0.0' is older than that of 
the runtime '3.1.0'. Update the tools for the latest 
features and bug fixes.

と言われたので最新版にする.

アップデートするコマンドを調べる(dotnet tool --help).

>dotnet tool --help
Usage: dotnet tool [options] [command]

Options:
  -h, --help   Show command line help.

Commands:
  install <PACKAGE_ID>     Install global or local tool. Local tools are added to manifest and restored.
  uninstall <PACKAGE_ID>   Uninstall a global tool or local tool.
  update <PACKAGE_ID>      Update a global tool.
  list                     List tools installed globally or locally.
  run <COMMAND_NAME>       Run local tool.
  restore                  Restore tools defined in the local tool manifest.

updateコマンドを使えばいい(dotnet tool update --global dotnet-ef).

>dotnet tool update --global dotnet-ef
dotnet tool update --global dotnet-ef
Tool 'dotnet-ef' was successfully updated from version '3.0.0' to version '3.1.0'.