Support `pex3 cache prune --older-than ...`. #3160

Job	Run time
Check GitHub Organization	1s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	8m 37s
tox -e py313-pip24_3_1	8m 43s
tox -e py313-pip24_3_1-integration --shard 1/2	7m 51s
./dtox.sh -e py27-pip20	8m 39s
tox -e py313-pip24_3_1-integration --shard 2/2	8m 43s
./dtox.sh -e py38-pip22_3_1	8m 39s
./dtox.sh -e py311-pip23_3_2	8m 41s
./dtox.sh -e py313-pip24_3_1	8m 38s
./dtox.sh -e py314-pip24_3_1	8m 40s
./dtox.sh -e pypy310-pip24_3_1	8m 39s
./dtox.sh -e py27-pip20-integration	8m 41s
./dtox.sh -e py38-pip22_3_1-integration --shard 1/2	8m 39s
./dtox.sh -e py38-pip22_3_1-integration --shard 2/2	8m 39s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	8m 39s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	8m 38s
./dtox.sh -e py313-pip24_3_1-integration --shard 1/2	8m 38s
./dtox.sh -e py313-pip24_3_1-integration --shard 2/2	8m 39s
./dtox.sh -e py314-pip24_3_1-integration --shard 1/2	8m 39s
./dtox.sh -e py314-pip24_3_1-integration --shard 2/2	8m 39s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 1/2	1s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 2/2	8m 39s
Gather Final Status	1s
	2h 52m 23s

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