fix(codemods): Use .cjs extension for CommonJS files (#7333)

* bug(codemods): rename CommonJS files to .cjs

Rename CommonJS files in codemods to distinguish them from ES module
files otherwise used in this project.
Resolve an issue occuring since package.json in codemods is removed when
publishing causing module type data to be lost.

Fix issue #7154

* Fix knip errors

* Add file extension to tests

* Update migration doc

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Co-authored-by: Lachlan Collins <1667261+lachlancollins@users.noreply.github.com>