SAML: Do not SAML SLO if user is not SAML authenticated (#53418)

* Only SLO user if the user is using SAML

* only one source of truth for auth module info

* ensure SAML is also enabled and not only SLO

* move auth module naming to auth module login package

* use constants in other previously unused spots

pkg/services/ldap/ldap.go

@@ -17,6 +17,7 @@ import (
 
 	"github.com/grafana/grafana/pkg/infra/log"
 	"github.com/grafana/grafana/pkg/models"
+	"github.com/grafana/grafana/pkg/services/login"
 )
 
 // IConnection is interface for LDAP connection manipulation
@@ -429,7 +430,7 @@ func (server *Server) buildGrafanaUser(user *ldap.Entry) (*models.ExternalUserIn
 
 	attrs := server.Config.Attr
 	extUser := &models.ExternalUserInfo{
-		AuthModule: models.AuthModuleLDAP,
+		AuthModule: login.LDAPAuthModule,
 		AuthId:     user.DN,
 		Name: strings.TrimSpace(
 			fmt.Sprintf(