chore(release): bump core to 6.4.0

[odin-posthog]

💡 Motivation and Context

💚 How did you test it?

📝 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.
• If there are related changes in the core package, I've already released them, or I'll release them before this one.

[github-actions]

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

Instructions and example for changelog

Please add an entry to the appropriate changelog:

• posthog/CHANGELOG.md (core module)
• posthog-android/CHANGELOG.md (android module)
• posthog-server/CHANGELOG.md (server module)
• posthog-android-gradle-plugin/CHANGELOG.md (android plugin module)

Make sure the entry includes this PR's number.
Example:

## Next
- bump core to 6.4.0 ([#397](https://github.com/PostHog/posthog-android/pull/397))

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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