Support `pex3 cache prune --older-than ...`. #3067

Job	Run time
Check GitHub Organization	1s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	8m 32s
tox -e py312-pip24_2	12m 59s
tox -e py312-pip24_2-integration --shard 1/2	37m 15s
./dtox.sh -e py27-pip20	10m 9s
./dtox.sh -e py37-pip22_3_1	12m 13s
tox -e py312-pip24_2-integration --shard 2/2	33m 57s
./dtox.sh -e py311-pip23_3_2	13m 36s
./dtox.sh -e py312-pip24_2	12m 35s
./dtox.sh -e py313-pip24_2	11m 2s
./dtox.sh -e pypy310-pip24_2	20m 25s
./dtox.sh -e py27-pip20-integration	27m 5s
./dtox.sh -e py37-pip22_3_1-integration --shard 1/2	25m 26s
./dtox.sh -e py37-pip22_3_1-integration --shard 2/2	28m 48s
./dtox.sh -e py311-pip23_3_2-integration --shard 1/2	28m 39s
./dtox.sh -e py311-pip23_3_2-integration --shard 2/2	26m 56s
./dtox.sh -e py312-pip24_2-integration --shard 1/2	29m 18s
./dtox.sh -e py312-pip24_2-integration --shard 2/2	27m 3s
./dtox.sh -e py313-pip24_2-integration --shard 1/2	24m 31s
./dtox.sh -e py313-pip24_2-integration --shard 2/2	27m 7s
./dtox.sh -e pypy310-pip24_2-integration --shard 1/2	45m 24s
./dtox.sh -e pypy310-pip24_2-integration --shard 2/2	46m 23s
Gather Final Status	1s
	8h 29m 25s

Provide feedback