Option to disable MkDocs 2.0 warning from mkdocs.yml

### Description We are using MkDocs 1.6 and for the time being have no plans or bandwidth to migrate. While I appreciate the heads up about MkDocs 2.0, for our day to day usage it is just noise and I would like to disable it. I know you can set the `NO_MKDOCS_2_WARNING` environment variable, but asking everybody who works on our site to manually set an environment variable gets tedious fast. It would be really nice if there was some way to disable the warning from `mkdocs.yml` or some other config file that can be commited to Git.