[3.50] Add Flutter 3.50.0 CHANGELOG.md entry - #195915

Adds the ## Flutter 3.50 Changes / ### [3.50.0] release notes and blog post links to CHANGELOG.md on flutter-3.50-candidate.0 per initial stable release guidance (go/flutter-release-workflow Step 6).

Part of #195830.

Wording Ownership

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.