Refs #20910 -- Replaced snippet directive with code-block.

docs/howto/initial-data.txt

@@ -58,7 +58,7 @@ look like in JSON:
 
 And here's that same fixture as YAML:
 
-.. code-block:: none
+.. code-block:: yaml
 
     - model: myapp.person
       pk: 1

docs/howto/writing-migrations.txt

@@ -39,8 +39,8 @@ attribute::
 You can also provide hints that will be passed to the :meth:`allow_migrate()`
 method of database routers as ``**hints``:
 
-.. snippet::
-    :filename: myapp/dbrouters.py
+.. code-block:: python
+    :caption: myapp/dbrouters.py
 
     class MyRouter:
 
@@ -97,8 +97,8 @@ the respective field according to your needs.
   of the three new files) to the last migration, change ``AddField`` to
   ``AlterField``, and add imports of ``uuid`` and ``models``. For example:
 
-  .. snippet::
-    :filename: 0006_remove_uuid_null.py
+  .. code-block:: python
+    :caption: 0006_remove_uuid_null.py
 
     # Generated by Django A.B on YYYY-MM-DD HH:MM
     from django.db import migrations, models
@@ -121,8 +121,8 @@ the respective field according to your needs.
 * Edit the first migration file. The generated migration class should look
   similar to this:
 
-  .. snippet::
-    :filename: 0004_add_uuid_field.py
+  .. code-block:: python
+    :caption: 0004_add_uuid_field.py
 
     class Migration(migrations.Migration):
 
@@ -148,8 +148,8 @@ the respective field according to your needs.
   unique value (UUID in the example) for each existing row. Also add an import
   of ``uuid``. For example:
 
-  .. snippet::
-    :filename: 0005_populate_uuid_values.py
+  .. code-block:: python
+    :caption: 0005_populate_uuid_values.py
 
     # Generated by Django A.B on YYYY-MM-DD HH:MM
     from django.db import migrations
@@ -279,8 +279,8 @@ project anywhere without first installing and then uninstalling the old app.
 
 Here's a sample migration:
 
-.. snippet::
-    :filename: myapp/migrations/0124_move_old_app_to_new_app.py
+.. code-block:: python
+    :caption: myapp/migrations/0124_move_old_app_to_new_app.py
 
     from django.apps import apps as global_apps
     from django.db import migrations