Rename migration lb_stats_needs_bigint to match revision number

This patch renames the migration 77dfe0962bb3_lb_stats_needs_bigint so
that it matches the revision number abc88c33f74f_lb_stats_needs_bigint

Change-Id: If07fd5d0cec3e37e7dfb0c1e71dedc111d7870be

diff --git a/neutron/db/migration/alembic_migrations/versions/77dfe0962bb3_lb_stats_needs_bigint.py b/neutron/db/migration/alembic_migrations/versions/abc88c33f74f_lb_stats_needs_bigint.py
similarity index 100%
rename from neutron/db/migration/alembic_migrations/versions/77dfe0962bb3_lb_stats_needs_bigint.py
rename to neutron/db/migration/alembic_migrations/versions/abc88c33f74f_lb_stats_needs_bigint.py