Support `pex3 cache prune --older-than ...`. #3140

Job	Run time
Check GitHub Organization	1s
tox -e py313-pip24_3	17m 35s
tox -e py313-pip24_3-integration --shard 1/2	22m 3s
./dtox.sh -e py27-pip20	9m 43s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	8m 52s
tox -e py313-pip24_3-integration --shard 2/2	33m 54s
./dtox.sh -e py38-pip22_3_1	18m 10s
./dtox.sh -e py311-pip23_3_2	19m 5s
./dtox.sh -e py313-pip24_3	14m 28s
./dtox.sh -e py314-pip24_3	14m 43s
./dtox.sh -e pypy310-pip24_3	27m 29s
./dtox.sh -e py27-pip20-integration	27m 36s
./dtox.sh -e py38-pip22_3_1-integration --shard 1/2	29m 41s
./dtox.sh -e py38-pip22_3_1-integration --shard 2/2	27m 37s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	28m 0s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	28m 58s
./dtox.sh -e py313-pip24_3-integration --shard 1/2	25m 9s
./dtox.sh -e py313-pip24_3-integration --shard 2/2	26m 15s
./dtox.sh -e py314-pip24_3-integration --shard 1/2	24m 22s
./dtox.sh -e py314-pip24_3-integration --shard 2/2	27m 19s
./dtox.sh -e pypy310-pip24_3-integration --shard 1/2	45m 32s
./dtox.sh -e pypy310-pip24_3-integration --shard 2/2	47m 6s
Gather Final Status	1s
	8h 43m 39s

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