[chore] Release 5.4.0 (#647)

* [chore] Release 5.4.0

Release v5.4.0
Deprecated Python 3.6 (reached EoL)

Author: lahirumaramba

firebase_admin/__about__.py

@@ -14,7 +14,7 @@
 
 """About information (version, etc) for Firebase Admin SDK."""
 
-__version__ = '5.3.0'
+__version__ = '5.4.0'
 __title__ = 'firebase_admin'
 __author__ = 'Firebase'
 __license__ = 'Apache License 2.0'