Fixes SSO login rejection (#2566)

# Description

## Checklist

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have performed a self-review of my own code
- [ ] I have attached images of the change if it is UI based
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] If my code has heavily changed functionality I have updated
relevant docs on [Stirling-PDFs doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
- [x] My changes generate no new warnings
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

src/main/resources/messages_en_GB.properties

@@ -561,6 +561,7 @@ login.oauth2invalidRequest=Invalid Request
 login.oauth2AccessDenied=Access Denied
 login.oauth2InvalidTokenResponse=Invalid Token Response
 login.oauth2InvalidIdToken=Invalid Id Token
+login.relyingPartyRegistrationNotFound=No relying party registration found
 login.userIsDisabled=User is deactivated, login is currently blocked with this username. Please contact the administrator.
 login.alreadyLoggedIn=You are already logged in to
 login.alreadyLoggedIn2=devices. Please log out of the devices and try again.