[google_maps_flutter] Add a pinned-SDK web example app

[stuartmorgan-g]

The tree is currently closed because Google Maps SDK 3.65 came out, and it throws exceptions why trying to use heatmaps, as that functionality has been removed from the SDK.

This fixes the tree by creating a second example app for google_maps_flutter_web that pins version 3.64 of the Google Maps SDK:

• copied the original example
• removed all of the non-heatmap tests
• consolidated the tests into one file (since it's not clear to me why we had two almost identically-named tests in the firs place, and it was especially confusing with just a few tests)

The heatmap tests have been removed from the main example, where they throw.

The README has been updated to explain that heatmaps are deprecated, and using them requires pinning 3.64.

Fixes flutter/flutter#186946

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• 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 followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception 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

[gemini-code-assist]

Code Review

This pull request updates the google_maps_flutter_web package to address the removal of heatmap support in version 3.65 of the Google Maps JavaScript SDK. Key changes include updating the README with deprecation notices and instructions for pinning the SDK to version 3.64, as well as restructuring the example apps into latest and 3-64 directories. Heatmap integration tests have been moved to the pinned 3-64 example. Review feedback includes corrections for a typo in the documentation, fixing inaccurate comments in the test files, and removing unused dependencies in the new example's configuration.

[stuartmorgan-g]

@bparrishMines @tarrinneal If either of you have a chance to review this before @mdebbar please take a look, since it will let us re-open the tree.