Upgrade mitchellh/hashstructure

https://github.com/mitchellh/hashstructure released a V2 which fixes (https://github.com/mitchellh/hashstructure/issues/18, https://github.com/mitchellh/hashstructure/issues/20). If we don't persist the hashes anywhere it should be safe to upgrade. It sounds like there is a way to get the old V1 hashes if we do persist them somewhere.

Current usage:
```
agent/agent_endpoint.go: rawHash, err := hashstructure.Hash(reply, nil)
agent/cache-types/connect_ca_leaf.go: v, err := hashstructure.Hash([]interface{}{
agent/cache-types/service_checks.go: v, err := hashstructure.Hash([]interface{}{
agent/cache-types/service_checks.go: b, err := hashstructure.Hash(wrapper, nil)
agent/consul/discoverychain/compile.go: v, err := hashstructure.Hash(customization, nil)
agent/structs/config_entry.go: v, err := hashstructure.Hash([]interface{}{
agent/structs/config_entry.go: v, err := hashstructure.Hash(struct {
agent/structs/config_entry_discoverychain.go: v, err := hashstructure.Hash(struct {
agent/structs/intention.go: v, err := hashstructure.Hash(q.Match, nil)
agent/structs/prepared_query.go: v, err := hashstructure.Hash([]interface{}{
agent/structs/structs.go: v, err := hashstructure.Hash([]interface{}{
agent/structs/structs.go: v, err := hashstructure.Hash([]interface{}{
agent/structs/structs.go: v, err := hashstructure.Hash([]interface{}{
agent/structs/structs.go: v, err := hashstructure.Hash([]interface{}{
```