FindBoost: Make imported targets fall back to `Release`

FindBoost only detects Debug and Release configurations.  All other
configurations will fall back to the configuration listed as the first
entry in `IMPORTED_CONFIGURATIONS`.  Switch the order so that `Release`
is listed first, as this is a better fallback than `Debug` for the
`RelWithDebInfo` and `MinSizeRel` configurations.  See issue #16091.