Convert remaining uses of gracefulswitch to use `gracefulswitch.ParseConfig` and not `SwitchTo`

[dfawley]

Follow-up work for #7035: convert other uses of this package.

When that is done, delete the SwitchTo method on gracefulswitch.Balancer (keeping switchTo) and also delete BalancerConfig:

https://github.com/grpc/grpc-go/blob/800a8e02b5f25fa6aceb5771c0c263dae0928f8f/internal/serviceconfig/serviceconfig.go#L42

[silaselisha]

@dfawley a polite request to be assigned this task.

[dfawley]

You're welcome to attempt it, but it could be tricky. Something from this list might be a little more new-to-grpc friendly: https://github.com/grpc/grpc-go/issues?q=is%3Aopen+label%3A%22Status%3A+Help+Wanted%22+sort%3Aupdated-desc