chore: flutter v4

[marandaneto]

:bulb: Motivation and Context

:green_heart: How did you test it?

:pencil: Checklist

• [ ] I reviewed the submitted code.
• [ ] I added tests to verify the changes.
• [ ] I updated the docs if needed.
• [ ] No breaking change or entry added to the changelog.

[github-actions[bot]]

	Fails
:no_entry_sign:	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Next" section. Make sure the entry includes this PR's number. Example:

  ## Next
  - flutter v4 ([#80](https://github.com/PostHog/posthog-flutter/pull/80))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

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