Remove BottomNavigationBarItem.title deprecation

[Piinks]

This removes the title parameter from BottomNavigationBarItem.

The replacement for which is to use label instead. This is supported by dart fix.

This deprecations has expired, as laid out in flutter.dev/go/deprecations-removed-after-2-5

Part of #90287

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[flutter-dashboard]

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[Piinks]

@HansMuller can you consult on the new API? I am not sure if label should be required now.

Update: It should not be required because the Cupertino tab bar does not require it, it has a test verifying this. The material BottomNavigationBar asserts that the BottomNavigationBarItems do have a label, so it is all covered.

[Piinks]

Just waiting on an internal change to land, there were some lingering dart doc references.

[Piinks]

Customers are migrated, this can land now.

[Piinks]

@goderbauer can you approve this now that everyone is migrated?

[goderbauer]

LGTM