[shared_preferences] Bump minimum Flutter version and iOS deployment target

[jmagman]

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

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.

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

shared_preferences part of flutter/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 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].
• 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.

[stuartmorgan]

LGTM