Revert spanner version (#37305)

Author: damccorm

buildSrc/src/main/groovy/org/apache/beam/gradle/BeamModulePlugin.groovy

@@ -614,7 +614,7 @@ class BeamModulePlugin implements Plugin<Project> {
     def google_clients_version = "2.0.0"
     def google_cloud_bigdataoss_version = "2.2.26"
     // [bomupgrader] determined by: com.google.cloud:google-cloud-spanner, consistent with: google_cloud_platform_libraries_bom
-    def google_cloud_spanner_version = "6.105.0"
+    def google_cloud_spanner_version = "6.104.0"
     def google_code_gson_version = "2.10.1"
     def google_oauth_clients_version = "1.34.1"
     // [bomupgrader] determined by: io.grpc:grpc-netty, consistent with: google_cloud_platform_libraries_bom