fix(codemod): avoid overwriting existing packageManager field (#8551)
### Description
Fixes #8308
Previously we would attempt to update the `packageManager` field to
whatever version we found for the listed package manager based on the
current `PATH`/cwd. This PR removes that behavior and now trusts the
user if they have one listed in `package.json`.
With the adoption of `corepack` more often then not, we want the
contents of `package.json` to inform the binary used instead of the
other way around.
### Testing Instructions
Updated existing unit tests to no longer expect changes