Bump to 2.5.0, make compatible with Unity 6.0+

[aurimasc]

https://jira.unity3d.com/browse/PLAT-19430
No (intentional) functional changes.
Make package compatible with Unity 6.0 or later (previously was 2021.3 or later).
Package version increased to 2.5.0 (required min version bump due to Unity compatibility bump).
Increasing minimum supported Unity version will allow cleanup to be done as it means minimum supported Android version is now higher too. No changes for this made here, will be follow up.
Most changes in this PR are test project upgrades to Unity 6, CI updates to only test on supported versions.
The only code changes are Android-specific deletions of code behind version defines. Automated test should fully over those (did try build locally too).

[todi1856]

/crc

[u-pr]

The review identified concerns regarding test coverage, performance, and error handling:

• Test Coverage: Existing tests for Android manifest injection were removed during the refactor without clear replacements.
• Performance: JNI lookups in AndroidNotificationCenter could benefit from caching to avoid repeated overhead.
• Error Handling: Standard .NET exception types should be used for missing JNI fields and methods for better clarity.

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