chore: Default to RPC version `0.7`

[t00ts]

We're still serving the 0.6 RPC API at the root path /.

We should default to 0.7.

Additional notes:

• Currently, to serve 0.7 it must be set on startup via --rpc.root-version.
• From the client side, it can be accessed specifying it in the url: /rpc/v0_7