Build broken because latest Flutter needs new `intl`

[gnprice]

This will be fixed by #567 after that's revised and merged.

Until then, CI runs are failing with an error like this:

  Note: intl is pinned to version 0.19.0 by flutter_localizations
    from the flutter SDK.
  See https://dart.dev/go/sdk-version-pinning for details.

  Because zulip depends on flutter_localizations from sdk which
    depends on intl 0.19.0, intl 0.19.0 is required.
  So, because zulip depends on intl ^0.18.0, version solving failed.

  You can try the following suggestion to make the pubspec resolve:
  * Try upgrading your constraint on intl: flutter pub add intl:^0.19.0

The root cause here is:

• Pin a precise version of Flutter #15

For the moment until this broken build is fixed, the workaround is to run tools/check locally with a version of Flutter that's at least a few days old. In particular, the minimum version specified in pubspec.yaml should always work.