Flutter apps run on 2.4.0-0.0.pre and laterwill be upgraded to a minimum of iOS 9.0. Now that Flutter 2.5 has hit stable andiOS 8 support has been dropped, change the plugin minimum iOS version to9.0.

Bump the Flutter and dart version constraints to 2.5 and 2.14 respectively to enforce that the iOS 9.0 migration has happened on the app side, because an older 8.0 app won't build with a 9.0 plugin. I believe the last time this was done was a few months ago for null safety adoption#3324 (there's recent precedent).

Users on Flutter versions lower than the current stable 2.5 will not be able to upgrade this plugin past this version.

image_picker part offlutter/flutter#84198

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]. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does usedart 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.
• Iupdated 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.