How can I reclaim the partition's space from this shrunk volume?
This cannot be done in Disk Utility, or even while running the OS in the active partition.

I normally backup the partition to an external HD, then restore to the Mac.
This is easier if you have another OS, but you should be able to do it from the Recovery mode.

Another option is to remove the HD, and do it on another Mac.

There may be 3rd party utilities which can move partition, but none I have used let you move the active partition.