availableupdates: Unify capitalization when comparing architectures

Otherwise, the following condition in cincinnati.go in MultiArch clusters:

```
if desiredArch == string(configv1.ClusterVersionArchitectureMulti) && currentArch != desiredArch {
    return current, []configv1.Release{current}, nil, nil
}
```

gets evaluated to:

```
if "Multi" == string(configv1.ClusterVersionArchitectureMulti) && "multi" != "Multi" {
    return current, []configv1.Release{current}, nil, nil
}
```

This will cause MultiArch clusters with a set non-empty
`ClusterVersion.Spec.DesiredUpdate.Architecture` field to indefinitely
have available updates set to `[]configv1.Release{current}` because
the `"multi" != "Multi"` logic will always match.

Author: DavidHurta

pkg/cvo/availableupdates.go

@@ -52,7 +52,7 @@ func (optr *Operator) syncAvailableUpdates(ctx context.Context, config *configv1
 	currentArch := runtime.GOARCH
 
 	if optr.release.Architecture == configv1.ClusterVersionArchitectureMulti {
-		currentArch = "multi"
+		currentArch = string(configv1.ClusterVersionArchitectureMulti)
 	}
 
 	if desiredArch == "" {