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.

_{🤖 Helpful? 👍/👎}

[D4n13l1u5]

Checked some basic functionality on Android and iOS:
On iOS:

• Checked that scheduled notifications with picture/audio, repeatable notifications are shown after the scheduled time passes
• Checked that the permission pop-ups show up for authorization sound + bad and alert

Android:

• Checked that new notification channels can be created (checked with fancy and default notification channels and checked that they show up when the “List All Channels” button is pressed)
• Checked that Exact scheduling can be requested, enabled, and notifications are more accurate compared to when exact scheduling is not enabled (exact scheduling: fire time: 12:57:31 -> received: 12:57:34, without exact scheduling: fire time: 12:56:48 -> received: 12:56:52)

Devices used: 
VLNQA00499 - Google Pixel 7 (Pixel 7), CPU: Google Tensor G2, GPU: Mali-G710, OS: 16
VLNQA00258 - iPhone Xs Max (MT502ET/A), CPU: Apple A12 Bionic, GPU: Apple designed, OS: 14.4