fix(hermetic_build): add grpc-gcp to module allowlist in root pom generator #2677

Job	Run time
filter	1m 6s
sdk-platform-java integration (25)	10m 8s
dependencies	3m 28s
unmanaged_dependency_check	12m 43s
clirr	3m 18s
sdk-platform-java units (8)	4m 41s
sdk-platform-java self-service clients (11)	7m 41s
sdk-platform-java integration (11)	9m 50s
sdk-platform-java self-service clients (17)	9m 4s
sdk-platform-java units (17)	4m 21s
sdk-platform-java units (21)	4m 11s
sdk-platform-java integration (17)	10m 18s
sdk-platform-java units (11)	4m 27s
sdk-platform-java units (25)	3m 58s
sdk-platform-java integration (21)	9m 8s
gapic-generator-java-bom	3m 12s
dependency_analyzer_unit_test	1m 22s
gapic-generator-java (8)	11m 48s
java8-compatibility	5m 59s
	2h 0m 43s

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