Cast astropy time MJD to float from np.float64

In numpy 2 sqlalchemy refuses to handle np.float64 and so the times
must be converted to Python float.

python/lsst/daf/butler/registry/obscore/_records.py

@@ -181,10 +181,10 @@ def __call__(self, ref: DatasetRef) -> Record | None:
         if timespan is not None:
             if timespan.begin is not None:
                 t_min = cast(astropy.time.Time, timespan.begin)
-                record["t_min"] = t_min.mjd
+                record["t_min"] = float(t_min.mjd)
             if timespan.end is not None:
                 t_max = cast(astropy.time.Time, timespan.end)
-                record["t_max"] = t_max.mjd
+                record["t_max"] = float(t_max.mjd)
 
         region = dataId.region
         if self.exposure.name in dataId: