notification_trigger: Rename private_message to direct_message.

This commit renames the 'PRIVATE_MESSAGE' attribute of the
'NotificationTriggers' class to 'DIRECT_MESSAGE'.

Custom migration to update the existing value in the database.

It includes 'TODO/compatibility' code to support the old
notification trigger value 'private_message' in the
push notification queue during the Zulip server upgrades.

Earlier 'private_message' was one of the possible values for the
'trigger' property of the '[`POST /zulip-outgoing-webhook`]' response;
Update the docs to reflect the change in the above-mentioned trigger
value.

zerver/migrations/0465_backfill_scheduledmessagenotificationemail_trigger.py

@@ -0,0 +1,24 @@
+# Generated by Django 4.2.3 on 2023-08-04 14:08
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0464_remove_realmplayground_url_prefix"),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            """
+            UPDATE zerver_scheduledmessagenotificationemail
+            SET trigger = 'direct_message'
+            WHERE trigger = 'private_message';
+            """,
+            reverse_sql="""
+            UPDATE zerver_scheduledmessagenotificationemail
+            SET trigger = 'private_message'
+            WHERE trigger = 'direct_message';
+            """,
+        ),
+    ]