Protobuf v3

[nathanielheitsch]

Small change, upgrades Protobuf to v3 and updates the example project to fix the deprecation of applying gradle plugins according to the documentation: https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply

[vsiddharthv]

Can you please fix this issue and merge changes? Because of protobuf v3 dependency in another plugin I use, it has created a deadlock. If flutter_reactive_ble supports protobuf v3, then the deadlock will be solved.

Thanks in advance!

> flutter pub get
Resolving dependencies... (1.1s)
Because reactive_ble_mobile 5.3.1 depends on protobuf ^2.0.0 and no versions of reactive_ble_mobile match >5.3.1 <6.0.0, reactive_ble_mobile ^5.3.1 requires protobuf ^2.0.0.
And because barcode_scan2 >=4.3.0 depends on protobuf ^3.1.0, barcode_scan2 >=4.3.0 is incompatible with reactive_ble_mobile ^5.3.1.
And because flutter_reactive_ble 5.3.1 depends on reactive_ble_mobile ^5.3.1 and no versions of flutter_reactive_ble match >5.3.1 <6.0.0, barcode_scan2 >=4.3.0 is incompatible with flutter_reactive_ble ^5.3.1.
So, because {my-app} depends on both flutter_reactive_ble ^5.3.1 and barcode_scan2 4.3.3, version solving failed.