Backport route replacement

[kdorosh]

backport of #3833

Description

Route replacement with UpstreamGroups does not work, allowing invalid config being sent to envoy.
Specifically, when route replacement is enabled and a route references a missing UpstreamGroup, an envoy Route will be served which does not contain a destination cluster of any type.

This change ensures that the route which points to a missing UpstreamGroup will be considered for when route replacement occurs.

Context

This bug is due to the fact that route replacement relies on the ClusterSpecifier of a Route being set, even if it references a missing or otherwise invalid cluster.

It wasn't working because if an UpstreamGroup isn't found, we immediately return an error which means a ClusterSpecifier is not set on the Route, e.g. it is nil

Checklist:

• I included a concise, user-facing changelog (for details, see https://github.com/solo-io/go-utils/tree/master/changelogutils) which references the issue that is resolved.
• If I updated APIs (our protos) or helm values, I ran make install-go-tools generated-code to ensure there will be no code diff
• I followed guidelines laid out in the Gloo contribution guide
• I opened a draft PR or added the work in progress label if my PR is not ready for review
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
  BOT NOTES:
  resolves Route Replacement does not work with UpstreamGroups #3818

[solo-changelog-bot]

Issues linked to changelog:
#3818