Add Envoy Gateway 1.2.1 (#478)

* add support for envoy-gateway 1.1.0

* Add Envoy Gateway 1.2.1

Author: vtomasr5

libs/envoy-gateway/config.jsonnet

@@ -1,5 +1,5 @@
 local config = import 'jsonnet/config.jsonnet';
-local versions = ['v0.6.0', 'v1.0.1', 'v1.1.0'];
+local versions = ['v0.6.0', 'v1.0.1', 'v1.1.0', 'v1.2.1'];
 
 config.new(
   name='envoy-gateway',