docs: update recommend node version (#182)

Fixes https://github.com/microsoft/vscode-test/issues/181

README.md

@@ -6,7 +6,7 @@ This module helps you test VS Code extensions.
 
 Supported:
 
-- Node >= 12.x
+- Node >= 16.x
 - Windows >= Windows Server 2012+ / Win10+ (anything with Powershell >= 5.0)
 - macOS
 - Linux