Removes outdated/unused version and updating code

I had the bright idea of creating this update script but it doesn't work
and hasn't been maintained, so it's just sitting there causing confusing
and requiring weird things to exist in other places.

Now, the unused `version` field can be removed and I can scrap the
management command for getting versions.

bookwyrm/migrations/0192_rename_version_sitesettings_available_version.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.23 on 2024-01-02 19:36
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('bookwyrm', '0191_merge_20240102_0326'),
+    ]
+
+    operations = [
+        migrations.RenameField(
+            model_name='sitesettings',
+            old_name='version',
+            new_name='available_version',
+        ),
+    ]