[v9]: Replace custom Platform & PlatformChecker with package platform

[denrase]

📜 Description

Replace Platform

💡 Motivation and Context

Relates to #2646

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[github-actions]

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

Instructions and example for changelog

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

Example:

## Unreleased

- Replace custom Platform & PlatformChecker with package platform ([#2729](https://github.com/getsentry/sentry-dart/pull/2729))

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

Generated by 🚫 dangerJS against b80690d

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.12%. Comparing base (75e3eba) to head (b80690d).
Report is 1 commits behind head on v9.

Additional details and impacted files

@@            Coverage Diff             @@
##               v9    #2729      +/-   ##
==========================================
- Coverage   88.50%   87.12%   -1.38%     
==========================================
  Files         262      121     -141     
  Lines        8758     4172    -4586     
==========================================
- Hits         7751     3635    -4116     
+ Misses       1007      537     -470     

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