Fix compilation issue in FCM module

[hborisoff]

What	Where/Who
People Involved	@hborisoff

Description of the changes:

1. Increased Gradle plugin version because of issues in Android Studio 4.2

2. Put upper bound to firebase-messaging dependency, because 22.0.0 removes some deprecated classes and causes compilation errors.

Recommended change for supporting 22.0.0 is to use

FirebaseMessaging.getInstance().getToken()
instead of
FirebaseInstanceId.getInstance().getInstanceId()

and

FirebaseMessaging.getInstance().deleteToken() or FirebaseInstallations.getInstance().delete()
instead of
FirebaseInstanceId.getInstance().deleteInstanceId()

but these methods are added in 20.3.0 and we need to wait a little before updating dependency to [20.3.0,), because it is released in Oct 2020, half a year ago.

List with all firebase-messaging versions.