[hotfix] Fix broken stable docs URL in outdated/unreleased banner (#1092)

The "stable version" link in the outdated/unreleased version banner
pointed to the legacy ci.apache.org host, which now 404s. The rest of
the repo already uses nightlies.apache.org; update the banner to match.

Author: spuru9

docs/layouts/partials/docs/inject/content-before.html

@@ -23,14 +23,14 @@
 {{ if $.Site.Params.ShowOutDatedWarning }}
 <article class="markdown">
     <blockquote style="border-color:#f66">
-        {{ markdownify "This documentation is for an out-of-date version of Apache Flink Kubernetes Operator. We recommend you use the latest [stable version](https://ci.apache.org/projects/flink/flink-kubernetes-operator-docs-stable/)."}}
+        {{ markdownify "This documentation is for an out-of-date version of Apache Flink Kubernetes Operator. We recommend you use the latest [stable version](https://nightlies.apache.org/flink/flink-kubernetes-operator-docs-stable/)."}}
     </blockquote>
 </article>
 {{ end }}
 {{ if (not $.Site.Params.IsStable) }}
 <article class="markdown">
     <blockquote style="border-color:#f66">
-        {{ markdownify "This documentation is for an unreleased version of the Apache Flink Kubernetes Operator. We recommend you use the latest [stable version](https://ci.apache.org/projects/flink/flink-kubernetes-operator-docs-stable/)."}}
+        {{ markdownify "This documentation is for an unreleased version of the Apache Flink Kubernetes Operator. We recommend you use the latest [stable version](https://nightlies.apache.org/flink/flink-kubernetes-operator-docs-stable/)."}}
     </blockquote>
 </article>
 {{ end }}