[chore] Release 6.0.0 (#649)

* [chore] Release 6.0.0

- Major version bump to drop support for Python 3.6
- Added `app_check.verify_token()` API

Author: lahirumaramba

firebase_admin/__about__.py

@@ -14,7 +14,7 @@
 
 """About information (version, etc) for Firebase Admin SDK."""
 
-__version__ = '5.4.0'
+__version__ = '6.0.0'
 __title__ = 'firebase_admin'
 __author__ = 'Firebase'
 __license__ = 'Apache License 2.0'