fix(java-firestore): preserve FirestoreAdminVersion.java during regeneration #19799

Job	Run time
package_name_check	55s
root-pom	1m 18s
group_id_check_for_maps_libraries	8m 32s
owlbot-py	1m 51s
owlbot-yaml	53s
gapic-bom	1m 26s
gitignore_correctness	50s
generate-readme	1m 6s
	16m 51s

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