fix(bigtable): prune older AFEs first (#13215) #20751

Job	Run time
generation-config-check	1m 43s
enforcer	19m 56s
changes	1m 22s
windows	2m 55s
lint	2m 56s
gapic-libraries-bom	18m 54s
bulk-filter	1m 21s
units (8)	46s
units (25)	37s
units (17)	29s
units (11)	30s
units (21)	32s
split-dependencies (java-bigtable)	5m 4s
split-clirr (java-bigtable)	2m 26s
split-units (java-bigtable, 8)	6h 0m 14s
split-units (java-bigtable, 25)	7m 49s
split-units (java-bigtable, 21)	7m 42s
split-units (java-bigtable, 11)	6m 19s
split-units (java-bigtable, 17)	7m 55s
conditional-required-check	3s
	7h 29m 33s

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