Unpin `package:intl`

[mosuem]

Fixes #158050

Created by running flutter update-packages --cherry-pick=intl:^0.20.2 --exclude-tools - we probably don't want to unpin flutter_tools dependencies to avoid breaking it.

Pre-launch Checklist

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

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