fix: resolve alembic multiple-heads conflict in migration chain

down_revision on d4e5f6a7b8c9 still pointed at 3a8e6f2c1b95, which
master's add_subjects_tables migration (b1c2d3e4f5a6) also forked
from, leaving two alembic heads and breaking `superset db upgrade`
in every DB-backed CI job.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: rusackas

superset/migrations/versions/2026-07-02_21-00_d4e5f6a7b8c9_migrate_bubble_chart_to_echarts.py

@@ -17,7 +17,7 @@
 """migrate_bubble_chart_to_echarts
 
 Revision ID: d4e5f6a7b8c9
-Revises: 3a8e6f2c1b95
+Revises: b1c2d3e4f5a6
 Create Date: 2026-07-02 21:00:00.000000
 
 """
@@ -31,7 +31,7 @@
 
 # revision identifiers, used by Alembic.
 revision = "d4e5f6a7b8c9"
-down_revision = "3a8e6f2c1b95"
+down_revision = "b1c2d3e4f5a6"
 
 
 def upgrade():