build: stop managing java-showcase Version.java #18221

Job	Run time
gapic-bom	1m 47s
generate-readme	1m 13s
owlbot-yaml	1m 8s
gitignore_correctness	1m 9s
owlbot-py	1m 12s
package_name_check	1m 4s
group_id_check_for_maps_libraries	8m 28s
root-pom	1m 40s
	17m 41s

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