[Docs] Update supported android versions in readme to include Android 14

[jkasten2]

Description

One Line Summary

Updated tested / supported Android versions in repos readme.

Details

Motivation

Developer use the readme to evaluate libraries before using, it is import Android versions are up to date so they know what is supported.

Scope

Only update Android versions in the Readme.

History

We support Android 14 (API 34) for a while but this readme was never updated. Also since OneSignal-Android-SDK 5.0.0 we only support Android 5.0 (API 21).

Testing

Unit testing

N/A

Manual testing

N/A

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

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This change is