Add supported kv engine version information to vault secret store

**What content needs to be created or modified?**
https://docs.dapr.io/reference/components-reference/supported-secret-stores/hashicorp-vault/

**Describe the solution you'd like**

Add The information that the supported kv engine version is `v2`, as well as how to enable a v2 engine.
Since this is not documented, new users may have no idea when runs into these issues. Such as dapr/components-contrib#2711