Bump keycloak-services from 13.0.0 to 15.1.1

Bumps [keycloak-services](https://github.com/keycloak/keycloak) from 13.0.0 to 15.1.1.
- [Release notes](https://github.com/keycloak/keycloak/releases)
- [Commits](keycloak/keycloak@13.0.0...15.1.1)

---
updated-dependencies:
- dependency-name: org.keycloak:keycloak-services
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

pom.xml

@@ -9,7 +9,7 @@
     <java.version>1.8</java.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <main.java.package>gr.cti.nts</main.java.package>
-    <keycloak.version>13.0.0</keycloak.version>
+    <keycloak.version>15.1.1</keycloak.version>
     <junit.version>4.8.1</junit.version>
     <keycloak.version>13.0.1</keycloak.version>
     <lombok.version>1.18.12</lombok.version>