chore: test bigtable CI for Java 8 #22427

Job	Run time
bulk-filter	1m 10s
lint	3m 40s
gapic-libraries-bom	21m 21s
enforcer	21m 18s
windows	1m 55s
changes	30s
units (8)	51s
units (17)	34s
units (21)	32s
units (25)	29s
units (11)	33s
units (26)	36s
split-clirr (java-bigtable)	2m 6s
split-units (java-bigtable, 8)	6m 11s
split-dependencies (java-bigtable)	4m 57s
split-units (java-bigtable, 26)	7m 19s
split-units (java-bigtable, 21)	6m 14s
split-units (java-bigtable, 17)	7m 44s
split-units (java-bigtable, 11)	6m 0s
split-units (java-bigtable, 25)	7m 40s
conditional-required-check	5s
	1h 41m 45s

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