lowercase DisplayName attributes when reading SSO response (#12545)

this is to accommodate providers like [Okta][1] that send the user's
full name as an attribute named `displayName`

[1]:
https://developer.okta.com/docs/reference/api/users/#default-profile-properties

changes/sso-display-name-case

@@ -0,0 +1 @@
+- Ignore casing in SAML response for display name. For example the display name attribute can be provided now as `displayname` or `displayName`.

server/sso/authorization_response.go

@@ -6,6 +6,7 @@ import (
 	"encoding/xml"
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/fleetdm/fleet/v4/server/fleet"
 )
@@ -108,7 +109,7 @@ func (r resp) UserID() string {
 func (r resp) UserDisplayName() string {
 	if r.response != nil {
 		for _, attr := range r.response.Assertion.AttributeStatement.Attributes {
-			if _, ok := validDisplayNameAttrs[attr.Name]; ok {
+			if _, ok := validDisplayNameAttrs[strings.ToLower(attr.Name)]; ok {
 				for _, v := range attr.AttributeValues {
 					if v.Value != "" {
 						return v.Value

server/sso/authorization_response_test.go

@@ -508,6 +508,7 @@ func TestUserDisplayName(t *testing.T) {
 	}{
 		{"name", []AttributeValue{{Value: "Name Surname"}}, "Name Surname"},
 		{"displayname", []AttributeValue{{Value: "Name Surname"}}, "Name Surname"},
+		{"displayName", []AttributeValue{{Value: "Name Surname"}}, "Name Surname"},
 		{"cn", []AttributeValue{{Value: "Name Surname"}}, "Name Surname"},
 		{"urn:oid:2.5.4.3", []AttributeValue{{Value: "Name Surname"}}, "Name Surname"},
 		{"http://schemas.xmlsoap.org/ws/2005/05/identity/claims/name", []AttributeValue{{Value: "Name Surname"}}, "Name Surname"},