[url_launcher_web] Update min Flutter version

[stuartmorgan-g]

The change in 2.0.7 requires Flutter 2.10, but the min SDK version
wasn't updated so it would break people who are still on 2.8 (or
earlier).

2.0.7 has been retracted on pub.dev to address that; this will
effectively re-release 2.0.7 with the right constraints.

(Also fixes heading formats in the changelog, since I noticed
that the display on pub.dev is currently very strange.)

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 relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart 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, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• 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.
• All existing and new tests are passing.

[stuartmorgan-g]

Test are failing because our master pin is too old :/ Hopefully we can resolve that today.

[ditman]

Test are failing because our master pin is too old :/ Hopefully we can resolve that today.

The new pin has landed a few minutes ago, I'm going to restart tests here.

We need to do this with PointerInterceptor too, even though I couldn't find any reports in the bug tracker.

[stuartmorgan-g]

The Flutter version on master is still too old. Apparently it hasn't been bumped yet.

[ditman]

I'll start the flutter roller and try again once the latest main lands into the repo.

[ditman]

Agh, jeez, I need to do this change AND rebase the branch. I'm dumb.

[ditman]

Didn't mention it earlier, but thanks for cleaning up the CHANGELOG too, it's going to look much better in pub.dev!