fix(place_tracker): Migrate to Dart 3 and resolve Null Safety crashes

[rabi-33]

Problem

The place_tracker sample was pinned to an outdated SDK version (>=2.7.0 <3.0.0) and used pre-Null Safety dependencies. This caused the following issues:

• Build Failure: The project could not be compiled on modern Flutter environments (Dart 3+).
• Runtime Crashes: The Maps_flutter version (0.5.x) utilized deprecated Android embedding v1 APIs.

Solution

This PR migrates the project to fully support Dart 3 and Flutter 3.x.

Key Changes:

1. SDK Upgrade: Bumped environment constraints to sdk: ">=3.0.0 <4.0.0".
2. Dependency Updates:
   • Maps_flutter: Upgraded to ^2.5.0 (fixes Android embedding warning).
   • provider: Upgraded to ^6.0.0.
   • go_router: Added for modern navigation handling.
3. Null Safety Migration:
   • Applied ? nullable types and late keywords throughout the codebase.
   • Added required null checks to prevent Null check operator used on a null value errors.

Verification

• Builds successfully on Flutter 3.19.x / Dart 3.3.
• Static Analysis: Passed flutter analyze with 0 errors.

Pre-launch Checklist

• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I read the [Contributors Guide].
• I have added sample code updates to the [changelog].
• I updated/added relevant documentation (doc comments with ///).

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[rabi-33]

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[rabi-33]

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