minor #16249 [Notifier] Remove Firebase transport from Push Channel list (ssitdikov)

This PR was submitted for the 6.0 branch but it was merged into the 5.4 branch instead.

Discussion
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[Notifier] Remove Firebase transport from Push Channel list

`FirebaseTransport` supports `ChatMessage` and not supports `TexterTransport` which indicated in Documentation.

Commits
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13ab7f6 Remove Firebase transport from Push

Author: javiereguiluz

notifier.rst

@@ -356,7 +356,6 @@ integration with these push services:
 ==============  ====================================  =================================================================================
 Service         Package                               DSN
 ==============  ====================================  =================================================================================
-Firebase        ``symfony/firebase-notifier``          ``firebase://USERNAME:PASSWORD@default``
 Expo            ``symfony/expo-notifier``              ``expo://Token@default``
 OneSignal       ``symfony/one-signal-notifier``        ``onesignal://APP_ID:API_KEY@default?defaultRecipientId=DEFAULT_RECIPIENT_ID''``
 ==============  ====================================  =================================================================================