document gotcha around third party plugins (#11032)
document gotcha around third party plugins (#11032)

* document gotcha around third party plugins
* Update website/content/docs/commands/hcl2_upgrade.mdx

Co-authored-by: Adrien Delorme <cd910f2c814b97a5a5de8374fb0c727053e37934@users.noreply.github.com>
pecigonzalo/packer,mitchellh/packer,pecigonzalo/packer,mitchellh/packer,hashicorp/packer,hashicorp/packer,hashicorp/packer,mitchellh/packer,hashicorp/packer,pecigonzalo/packer,mitchellh/packer,pecigonzalo/packer,mitchellh/packer