[JENKINS-73487] Fix Stapler exception with multiple security warnings (…

…jenkinsci#9983)

Co-authored-by: Daniel Beck <daniel-beck@users.noreply.github.com>
(cherry picked from commit 5a574c9)

core/src/main/resources/jenkins/security/UpdateSiteWarningsMonitor/message.groovy

@@ -42,7 +42,7 @@ def listWarnings(warnings, boolean core) {
             }
         }
     }
-    if (fixables == warnings.size) {
+    if (fixables == warnings.size()) {
         dd {
             if (fixables == 1) {
                 raw(_(core ? "allFixable1Core" : "allFixable1", rootURL))