[google_maps_flutter] Document tile cache flicker

[faheemabbas766]

Updates GoogleMapController.clearTileCache documentation to explain that some platform implementations may remove current tiles before replacements are available, which can cause the tile overlay to flicker. The wording remains implementation-neutral so it also applies to third-party platform implementations.

Fixes flutter/flutter#189678.

Pre-Review Checklist

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Test exemption: This PR changes only API documentation and the required release metadata; it does not change runtime behavior.

Validation:

• dart format --output=none --set-exit-if-changed packages/google_maps_flutter/google_maps_flutter/lib/src/controller.dart
• dart run script/tool/bin/flutter_plugin_tools.dart analyze --packages=google_maps_flutter/google_maps_flutter --exact-match-only
• dart run script/tool/bin/flutter_plugin_tools.dart dart-test --packages=google_maps_flutter/google_maps_flutter --exact-match-only
• dart run script/tool/bin/flutter_plugin_tools.dart validate --packages=google_maps_flutter/google_maps_flutter --exact-match-only --base-branch=upstream/main
• dart run script/tool/bin/flutter_plugin_tools.dart publish-check --packages=google_maps_flutter/google_maps_flutter --exact-match-only

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Footnotes

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[gemini-code-assist]

Code Review

This pull request updates the google_maps_flutter package to version 2.18.1. The changes update the documentation for the clearTileCache method in GoogleMapController to clarify that clearing the tile cache may cause the tile overlay to flicker on some platforms. The CHANGELOG.md and pubspec.yaml files have been updated accordingly. There are no review comments to address.