Update 6_0_upgrade.md (#2849)
Update 6_0_upgrade.md (#2849)

Depending on local installations, outdated versions of webpack-dev-server might not work with webpack 5. 

Uninstalling the current version is one way to help developers avoid running into issues when trying to run the dev server after upgrading since the install task will upgrade in this case.
rails/webpacker,rails/webpacker