iOS update for geocoding. Updates platform interface and minimal OS v…

Updates platform interface for iOS and updates Example project to minimal deployment target 12.

Partially fixes: 183

Pre-launch Checklist

[x] I made sure the project builds.
[x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is does not need version changes.
[x] I updated CHANGELOG.md to add a description of the change.
[x] I updated/added relevant documentation (doc comments with ///).
[x] I rebased onto main.
[x] I added new tests to check the change I am making, or this PR does not need tests.
[x] I made sure all existing and new tests are passing.
[x] I ran dart format . and committed any changes.
[x] I ran flutter analyze and fixed any errors.