chore(pubsub): migrate java-pubsub to monorepo #20438

Job	Run time
enforcer	19m 7s
windows	2m 8s
lint	3m 11s
bulk-filter	1m 3s
changes	4s
gapic-libraries-bom	19m 14s
generation-config-check	1m 37s
split-clirr (java-pubsub)	2m 24s
split-dependencies (java-pubsub)	2m 21s
split-units (java-pubsub, 17)	2m 44s
split-units (java-pubsub, 8)	6h 5m 0s
split-units (java-pubsub, 25)	2m 58s
split-units (java-pubsub, 21)	2m 34s
split-units (java-pubsub, 11)	2m 37s
units (8)	33m 56s
units (17)	32m 22s
units (21)	32m 49s
units (11)	34m 40s
units (25)	26m 28s
conditional-required-check	4s
	9h 47m 21s

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