Support `pex3 cache prune --older-than ...`. #3149

Job	Run time
Check GitHub Organization	1s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	8m 51s
./dtox.sh -e py27-pip20	9m 29s
tox -e py313-pip24_3_1	15m 28s
./dtox.sh -e py38-pip22_3_1	17m 54s
tox -e py313-pip24_3_1-integration --shard 1/2	23m 53s
./dtox.sh -e py311-pip23_3_2	18m 16s
tox -e py313-pip24_3_1-integration --shard 2/2	35m 34s
./dtox.sh -e py313-pip24_3_1	15m 9s
./dtox.sh -e py314-pip24_3_1	14m 32s
./dtox.sh -e pypy310-pip24_3_1	27m 59s
./dtox.sh -e py27-pip20-integration	27m 31s
./dtox.sh -e py38-pip22_3_1-integration --shard 1/2	31m 4s
./dtox.sh -e py38-pip22_3_1-integration --shard 2/2	27m 57s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	29m 31s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	28m 48s
./dtox.sh -e py313-pip24_3_1-integration --shard 1/2	26m 23s
./dtox.sh -e py313-pip24_3_1-integration --shard 2/2	26m 16s
./dtox.sh -e py314-pip24_3_1-integration --shard 1/2	24m 18s
./dtox.sh -e py314-pip24_3_1-integration --shard 2/2	27m 29s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 1/2	43m 34s
./dtox.sh -e pypy310-pip24_3_1-integration --shard 2/2	47m 40s
Gather Final Status	1s
	8h 47m 38s

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