Add ACS Location Default

Closes gh-8876

Author: jzheaux

saml2/saml2-service-provider/src/main/java/org/springframework/security/saml2/provider/service/registration/RelyingPartyRegistration.java

@@ -28,6 +28,7 @@
 import java.util.function.Function;
 
 import org.springframework.security.saml2.core.Saml2X509Credential;
+import org.springframework.security.saml2.provider.service.servlet.filter.Saml2WebSsoAuthenticationFilter;
 import org.springframework.util.Assert;
 
 /**
@@ -715,7 +716,7 @@ public final static class Builder {
 		private String entityId = "{baseUrl}/saml2/service-provider-metadata/{registrationId}";
 		private Collection<Saml2X509Credential> signingX509Credentials = new HashSet<>();
 		private Collection<Saml2X509Credential> decryptionX509Credentials = new HashSet<>();
-		private String assertionConsumerServiceLocation;
+		private String assertionConsumerServiceLocation = "{baseUrl}" + Saml2WebSsoAuthenticationFilter.DEFAULT_FILTER_PROCESSES_URI;
 		private Saml2MessageBinding assertionConsumerServiceBinding = Saml2MessageBinding.POST;
 		private ProviderDetails.Builder providerDetails = new ProviderDetails.Builder();
 		private Collection<org.springframework.security.saml2.credentials.Saml2X509Credential> credentials = new HashSet<>();