Update keycloak/keycloak Docker tag to v26.6.0 (#2673)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

src/test/java/plugins/OicAuthPluginTest.java

@@ -45,7 +45,7 @@ public class OicAuthPluginTest extends AbstractJUnitTest {
     private static final String CLIENT = "jenkins";
 
     private static final String KEYCLOAK_IMAGE =
-            "keycloak/keycloak:26.5.7@sha256:45ae20191531eb608ddb0b775d012b40d3e4f942697f3214694887dd7c108d13";
+            "keycloak/keycloak:26.6.0@sha256:b0e5dbced1775de4d629f103c0a9cfc057decc62ce8d3cb1c54f8849a6c6eb62";
 
     private KeycloakContainer keycloak;