Support `pex3 cache prune --older-than ...`. #3150

Job	Run time
Check GitHub Organization	1s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	5m 2s
./dtox.sh -e py27-pip20	9m 24s
tox -e py313-pip24_3_1	16m 32s
./dtox.sh -e py38-pip22_3_1	18m 17s
tox -e py313-pip24_3_1-integration --shard 1/2	24m 21s
tox -e py313-pip24_3_1-integration --shard 2/2	24m 22s
./dtox.sh -e py311-pip23_3_2	18m 35s
./dtox.sh -e py313-pip24_3_1	14m 41s
./dtox.sh -e py314-pip24_3_1	15m 36s
./dtox.sh -e pypy310-pip24_3_1	24m 21s
./dtox.sh -e py27-pip20-integration	24m 22s
./dtox.sh -e py38-pip22_3_1-integration --shard 1/2	24m 20s
./dtox.sh -e py38-pip22_3_1-integration --shard 2/2	24m 22s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	24m 22s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	24m 20s
./dtox.sh -e py313-pip24_3_1-integration --shard 1/2	24m 22s
./dtox.sh -e py313-pip24_3_1-integration --shard 2/2	24m 20s
./dtox.sh -e py314-pip24_3_1-integration --shard 1/2	24m 19s
./dtox.sh -e py314-pip24_3_1-integration --shard 2/2	24m 21s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 1/2	14m 52s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 2/2	24m 21s
Gather Final Status	1s
	7h 9m 34s

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