chore: add license headers to java-showcase IT files #18228

Job	Run time
gitignore_correctness	1m 9s
root-pom	1m 48s
package_name_check	1m 5s
generate-readme	1m 13s
group_id_check_for_maps_libraries	8m 14s
owlbot-yaml	1m 8s
owlbot-py	1m 12s
gapic-bom	1m 52s
	17m 41s

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