[google_maps_flutter] Add README section about SDK versions

[stuartmorgan-g]

Updates the iOS section of the app-facing package's README to discuss the new google_maps_flutter_ios_sdk* packages and how to select between them.

Also removes the Obj-C part of the delegate update example, since at this point it's extremely unlikely that new developers are using Obj-C, and if they are they are making a deliberate choice to do so and so can be reasonably expected to know how to translate to Obj-C.

Fixes flutter/flutter#146920

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to 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 I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[gemini-code-assist]

Code Review

This pull request updates documentation for the google_maps_flutter package. The CHANGELOG.md and pubspec.yaml are updated to version 2.14.2. The primary changes are in README.md, where the iOS setup instructions have been revised. The Objective-C example for API key setup is removed, and a new, detailed section is added to explain how to select different Google Maps SDK versions for iOS. This new section covers the default behavior, how to select specific SDK versions, and provides important guidance for package authors. My feedback includes a suggestion to clarify the description of the default SDK selection behavior for improved accuracy.

[stuartmorgan-g]

I don't love how verbose this is, but I couldn't see a way to take anything out without removing context that's critical to understanding why this option even exists and how to navigate it. As the CocoaPods->SwiftPM transition progresses we can probably shorten it by assuming SwiftPM and just describing the SDK options, and then having the default implementation part be last as a more legacy discussion. (And eventually, when Flutter requires iOS 16, we can just remove that part entirely.)

I'm also not happy about adding more platform-specific docs to the app-facing README, since we are trying to stop doing that, but the new standard practice is to delegate to the platform implementation package README for that, and that would be weird here ("Go read the google_maps_flutter_ios README to read about not using google_maps_flutter_ios!") so this seems like the best option.