Dropping spanner tables which has row deletion policy

[toga4]

Is your feature request related to a problem? Please describe.
Currently, dropping will fail if spanner database has table with row deletion policy.

DDL:

CREATE TABLE Orders (
  OrderId INT64,
  CreatedAt TIMESTAMP,
) PRIMARY KEY (OrderId)
, ROW DELETION POLICY (OLDER_THAN(CreatedAt, INTERVAL 1 DAY))

Attempts to drop fail as follows:

% migrate -path db/migrations -database "spanner://projects/abc/instances/def/databases/testdb" down
Are you sure you want to drop the entire database schema? [y/N]
y
Dropping the entire database schema
error: rpc error: code = InvalidArgument desc = ROW DELETION POLICY will break if drop/alter column named CreatedAt in table Orders. Row Deletion Policy must be set on column of type TIMESTAMP. in line 0: DROP TABLE Orders; DROP TABLE SchemaMigrations

Describe the solution you'd like
Drop row deletion policy before dropping table which has row deletion policy.

Describe alternatives you've considered
Drop row deletion policy manually.