Fix wagtailembeds thumbnail_url migration for MySQL 8.0.13 (#6999)

* Upgrade mysql to 8.0.23 for github actions

* Fix thumbnail_url migration to work on Django 3 / MySQL 8.0.13+

Work around https://code.djangoproject.com/ticket/32503 by applying the not-null constraint before converting to TextField.

.github/workflows/test.yml

@@ -125,7 +125,7 @@ jobs:
 
     services:
       mysql:
-        image: mysql:5.7
+        image: mysql:8.0.23
         env:
           MYSQL_ALLOW_EMPTY_PASSWORD: yes
           MYSQL_DATABASE: wagtail

wagtail/embeds/migrations/0008_allow_long_urls.py

@@ -24,13 +24,24 @@ class Migration(migrations.Migration):
             name="url",
             field=models.TextField(),
         ),
+        # Converting URLField to TextField with a default specified (even with preserve_default=False)
+        # fails with Django 3.0 and MySQL >=8.0.13 (see https://code.djangoproject.com/ticket/32503) -
+        # work around this by altering in two stages, first making the URLField non-null then converting
+        # to TextField
         migrations.AlterField(
             model_name="embed",
             name="thumbnail_url",
-            field=models.TextField(
+            field=models.URLField(
                 blank=True,
                 default="",
             ),
             preserve_default=False,
         ),
+        migrations.AlterField(
+            model_name="embed",
+            name="thumbnail_url",
+            field=models.TextField(
+                blank=True,
+            ),
+        ),
     ]