Stop using floating alpha crossplane stack version

We were using a floating alpha version, which has seen a breaking
release since the chart was added to was added to GitLab. This broke
the integration setup in a hard-to-debug way (a deployment gets stuck in
a restart loop).

See https://gitlab.com/gitlab-org/gitlab/-/issues/210171

app/models/clusters/applications/crossplane.rb

@@ -49,8 +49,7 @@ module Clusters
         {
           "clusterStacks" => {
              self.stack => {
-               "deploy" => true,
-                  "version" => "alpha"
+               "deploy" => true
             }
           }
         }

changelogs/unreleased/use-default-crossplane-stack-versions.yml

+---
+title: Fix installation of GitLab-managed crossplane chart
+merge_request: 27040
+author:
+type: fixed