Support `pex3 cache prune --older-than ...`. #3154

Job	Run time
Check GitHub Organization	1s
tox -e py313-pip24_3_1	18m 46s
./dtox.sh -e py27-pip20	9m 22s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	8m 33s
tox -e py313-pip24_3_1-integration --shard 1/2	34m 45s
./dtox.sh -e py38-pip22_3_1	36m 59s
tox -e py313-pip24_3_1-integration --shard 2/2	33m 54s
./dtox.sh -e py311-pip23_3_2	20m 33s
./dtox.sh -e py313-pip24_3_1	33m 7s
./dtox.sh -e py314-pip24_3_1	14m 47s
./dtox.sh -e pypy310-pip24_3_1	28m 21s
./dtox.sh -e py27-pip20-integration	46m 33s
./dtox.sh -e py38-pip22_3_1-integration --shard 1/2	47m 54s
./dtox.sh -e py38-pip22_3_1-integration --shard 2/2	47m 36s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	27m 43s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	29m 18s
./dtox.sh -e py313-pip24_3_1-integration --shard 1/2	26m 32s
./dtox.sh -e py313-pip24_3_1-integration --shard 2/2	26m 31s
./dtox.sh -e py314-pip24_3_1-integration --shard 1/2	42m 50s
./dtox.sh -e py314-pip24_3_1-integration --shard 2/2	27m 1s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 1/2	51m 43s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 2/2	48m 39s
Gather Final Status	1s
	11h 1m 29s

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