chore(spanner): use channel affinity #19158

Job	Run time
owlbot-yaml	1m 11s
gitignore_correctness	1m 14s
gapic-bom	1m 44s
package_name_check	1m 18s
root-pom	1m 57s
generate-readme	1m 9s
group_id_check_for_maps_libraries	8m 41s
owlbot-py	1m 26s
	18m 40s

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