alembic: Synchronize alembic heads between supported branches.

This adds a dummy migration to 18 and 20 so that our alembic heads are
synchronized across all supported branches.

In this case the migration we are stubbing (24c12d8e9014) is:

775352ee6c
(cherry picked from commit 6c3eaba7c6)

contrib/ast-db-manage/config/versions/24c12d8e9014_dummy_migration.py

@@ -0,0 +1,25 @@
+"""dummy migration
+
+This migration exists to ensure that all currently supported branches
+have the same alembic revision head. This makes managing migrations
+across supported branches less painful.
+
+The migration that is stubbed here is:
+
+    https://github.com/asterisk/asterisk/commit/775352ee6c2a5bcd4f0e3df51aee5d1b0abf4cbe
+
+Revision ID: 24c12d8e9014
+Revises: 37a5332640e2
+Create Date: 2024-01-05 14:14:47.510917
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '24c12d8e9014'
+down_revision = '37a5332640e2'
+
+def upgrade():
+    pass
+
+def downgrade():
+    pass