requirements: Update social-auth-core to latest version.

Uses git release as this version 3.4.0 is not released to pypi.
This is required for removing some overriden functions of
apple auth backend class AppleAuthBackend.

With the update we also make following changes:

* Fix full name being populated as "None None".
c5c74f27dd that's included in update assigns first_name and last_name
to None when no name is provided by apple. Due to this our
code is filling return_data['full_name'] to 'None None'.
This commit fixes it by making first and last name strings empty.

* Remove decode_id_token override.
Python social auth merged the PR we sent including the changes
we made to decode_id_token function. So, now there is no
necessity for the override.

* Add _AUDIENCE setting in computed_settings.py.
`decode_id_token` is dependent on this setting.

zproject/computed_settings.py

@@ -47,6 +47,7 @@ from .configured_settings import (
     REMOTE_POSTGRES_SSLMODE,
     SENDFILE_BACKEND,
     SENTRY_DSN,
+    SOCIAL_AUTH_APPLE_BUNDLE_ID,
     SOCIAL_AUTH_APPLE_SERVICES_ID,
     SOCIAL_AUTH_GITHUB_KEY,
     SOCIAL_AUTH_GITHUB_ORG_NAME,
@@ -1028,6 +1029,8 @@ SOCIAL_AUTH_LOGIN_ERROR_URL = '/login/'
 # SERVICES_ID to make things more readable in the configuration
 # and our own custom backend code.
 SOCIAL_AUTH_APPLE_CLIENT = SOCIAL_AUTH_APPLE_SERVICES_ID
+SOCIAL_AUTH_APPLE_AUDIENCE = [id for id in [SOCIAL_AUTH_APPLE_CLIENT, SOCIAL_AUTH_APPLE_BUNDLE_ID] if id is not None]
+
 if PRODUCTION:
     SOCIAL_AUTH_APPLE_SECRET = get_from_file_if_exists("/etc/zulip/apple/zulip-private-key.key")
 else: