Downgrade node in antora build

- Mostly to make it easier for those who can't yet use node 18.

spring-shell-docs/spring-shell-docs.gradle

@@ -14,6 +14,10 @@ dependencies {
 	testImplementation 'org.awaitility:awaitility'
 }
 
+node {
+	version = '16.15.0'
+}
+
 antora {
 	version = '3.2.0-alpha.2'
 	options = [clean: true, fetch: !project.gradle.startParameter.offline, stacktrace: true]
@@ -42,4 +46,4 @@ def resolvedVersions(Configuration configuration) {
 	return configuration.resolvedConfiguration
 			.resolvedArtifacts
 			.collectEntries { [(it.name + '-version'): it.moduleVersion.id.version] }
-}
+}