xds: Store WeightedClusters as a slice instead of a map inside the Route (#8632)

xds: Store WeightedClusters as a slice instead of a map inside the Route (#8632)

Reasons for this change:
- The `WeightedClusters` field is never used as a map.
- Weighted clusters are stored as a list in the original envoy proto as
well.
- In tests that require deterministic WRR behavior, we use the
`testutils.NewTestWRR` to get rid of the randomness. But the output
still depends on the order in which items are added to the WRR. Maps in
Go are non-deterministic.

RELEASE NOTES: N/A