Support eslint 8

[sjdemartini]

Closes https://github.com/jens-ox/eslint-plugin-material-ui-unused-classes/issues/8

This is a PR for the branch mentioned here https://github.com/jens-ox/eslint-plugin-material-ui-unused-classes/issues/8#issuecomment-1025318418, which makes the suggested change to allow eslint 8.

@jens-ox would you be able to merge this in and release to allow this plugin to be used cleanly with newer versions of eslint as well?

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[sjdemartini]

This was separately resolved in the v2 version, so closing this out