website: remove UpCloud from manifest

Since the UpCloud plugin has moved to the integrations model, we don't
want to continue looking for a docs.zip in their repo, as it doesn't
exist anymore with this migration.

website/data/plugins-manifest.json

@@ -89,15 +89,6 @@
     "version": "latest",
     "pluginTier": "community"
   },
-  {
-    "title": "UpCloud",
-    "path": "upcloud",
-    "repo": "UpCloudLtd/packer-plugin-upcloud",
-    "version": "v1.5.2",
-    "pluginTier": "verified",
-    "sourceBranch": "master",
-    "isHcpPackerReady": true
-  },
   {
     "title": "Volcengine",
     "path": "volcengine",