fix(sensors_plus): Resolve incorrect cleanup method call

[kaismic]

Description

I'm using this plugin in my app, and I was testing my app on iOS and noticed that the console rapidly printed Communicating on a dead channel.
So after debugging, I figured out that FPPMagnetometerStreamHandlerPlus.onCancel was incorrectly calling _motionManager.stopDeviceMotionUpdates() instead of _motionManager.stopMagnetometerUpdates().

Related Issues

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[vbuberen]

Good catch. Thank you for this contribution. Seems like a leftover after #2250

Ignore failing example builds - it is not related to your changes and will be addressed separately.