api: Expose server_needs_upgrade in register response.

Expose the boolean value server_needs_upgrade in the
responses for register api so that it can be used
by mobile and terminal clients as well.
Highlighted in api changelog as part of
feature level 74 in commit fb93c96
(next commit).

zerver/lib/events.py

@@ -22,6 +22,7 @@ from zerver.lib.actions import (
 from zerver.lib.alert_words import user_alert_words
 from zerver.lib.avatar import avatar_url
 from zerver.lib.bot_config import load_bot_config_template
+from zerver.lib.compatibility import is_outdated_server
 from zerver.lib.external_accounts import DEFAULT_EXTERNAL_ACCOUNTS
 from zerver.lib.hotspots import get_next_hotspots
 from zerver.lib.integrations import EMBEDDED_BOTS, WEBHOOK_INTEGRATIONS
@@ -275,6 +276,8 @@ def fetch_initial_state_data(
         state["server_name_changes_disabled"] = settings.NAME_CHANGES_DISABLED
         state["giphy_rating_options"] = realm.GIPHY_RATING_OPTIONS
 
+        state["server_needs_upgrade"] = is_outdated_server(user_profile)
+
         # TODO: Should these have the realm prefix replaced with server_?
         state["realm_push_notifications_enabled"] = push_notifications_enabled()
         state["realm_default_external_accounts"] = DEFAULT_EXTERNAL_ACCOUNTS

zerver/lib/home.py

@@ -8,7 +8,6 @@ from django.http import HttpRequest
 from django.utils import translation
 from two_factor.utils import default_device
 
-from zerver.lib.compatibility import is_outdated_server
 from zerver.lib.events import do_events_register
 from zerver.lib.i18n import (
     get_and_set_request_language,
@@ -186,7 +185,6 @@ def build_page_params_for_home_page_load(
         test_suite=settings.TEST_SUITE,
         poll_timeout=settings.POLL_TIMEOUT,
         insecure_desktop_app=insecure_desktop_app,
-        server_needs_upgrade=is_outdated_server(user_profile),
         login_page=settings.HOME_NOT_LOGGED_IN,
         save_stacktraces=settings.SAVE_FRONTEND_STACKTRACES,
         warn_no_email=settings.WARN_NO_EMAIL,

zerver/openapi/zulip.yaml

@@ -8976,6 +8976,16 @@ paths:
                           Whether the server allows name changes.  Similar to
                           `realm_name_changes_disabled` but based on the `NAME_CHANGES_DISABLED`
                           Zulip server level setting.
+                      server_needs_upgrade:
+                        type: boolean
+                        description: |
+                          Present if `realm` is present in `fetch_event_types`.
+
+                          Whether the server is running an old version based on the Zulip
+                          [server release lifecycle](https://zulip.readthedocs.io/en/latest/overview/release-lifecycle.html#upgrade-nag),
+                          such that the web app will display to the current user a prominent warning.
+
+                          **Changes**: New in Zulip 5.0 (feature level 74).
                       realm_notifications_stream_id:
                         type: integer
                         description: |