Update keycloak/keycloak Docker tag to v26.5.4 (jenkinsci#2609)

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:26.5.2@sha256:fb31a59deb46f746f2aaa25adc5da39ceccac4fd22d36a519562b0bf02e8df20";
+            "keycloak/keycloak:26.5.4@sha256:ae8efb0d218d8921334b03a2dbee7069a0b868240691c50a3ffc9f42fabba8b4";
 
     private KeycloakContainer keycloak;