fix(firestore): increase ITShutdownTest timeout to 50s #19013

Job	Run time
group_id_check_for_maps_libraries	8m 28s
package_name_check	1m 6s
owlbot-py	37s
gitignore_correctness	27s
generate-readme	31s
root-pom	1m 12s
owlbot-yaml	1m 8s
gapic-bom	1m 6s
	14m 35s

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