Update Pyrra to v0.6.3

I want to deploy that version and rather than doing an overwrite on our end, I thought I might just send the PR through here right away.

jsonnet/kube-prometheus/versions.json

@@ -9,5 +9,5 @@
   "prometheusOperator": "0.65.1",
   "kubeRbacProxy": "0.14.1",
   "configmapReload": "0.5.0",
-  "pyrra": "0.6.2"
+  "pyrra": "0.6.3"
 }