Remove the managed flag from all existing EEs

This flag henceforth is going to be used only for the "control plane"
execution environments, which sysadmins will not be allowed to alter.

awx/main/migrations/0145_deregister_managed_ee_objs.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2.16 on 2021-06-07 19:36
+
+from django.db import migrations
+
+
+def forwards(apps, schema_editor):
+    ExecutionEnvironment = apps.get_model('main', 'ExecutionEnvironment')
+    for row in ExecutionEnvironment.objects.filter(managed_by_tower=True):
+        row.managed_by_tower = False
+        row.save(update_fields=['managed_by_tower'])
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0144_event_partitions'),
+    ]
+
+    operations = [
+        migrations.RunPython(forwards),
+    ]