[url_launcher] Fix breaking change version conflict

[creativecreatorormaybenot]

cc @Hixie @stuartmorgan I explained here why a breaking change was handled inproperly and this is why we need to specify the dependency that is at least 2.0.3.

Before that version, the method head of pushRouteNameToFramework was different, which means that ^2.0.0 for the platform interface is a lie - any version before 2.0.3 is incompatible.

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 and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

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[stuartmorgan-g]

The fix here is correct (there's an alternate option of making this actually backward compatible down to 2.0.0 again by changing the function variable type, but getting more people onto url_launcher_platform_interface 2.0.3 is a better outcome since 2.0.0-2 don't work correctly with master), but the description in the PR and the CHANGELOG are not, per #3954 (comment), unless I've missed something.

[creativecreatorormaybenot]

@stuartmorgan So you think this PR would be fine to merge if the CHANGELOG was amended. If so, what would you include in the changelog / how would you phrase it?

[stuartmorgan-g]

I've rewritten the changelog; LGTM with that. Thanks for finding this!

[creativecreatorormaybenot]

@stuartmorgan Thanks a lot ❤️