getHealth's retention window is broken

[2opremio]

It returns the transaction retention window, instead of the maximum retention window.

It broke in #174

In particular, this is not true:

https://github.com/stellar/soroban-rpc/blob/c818bb87eaaf705ca4b4c1f4b5ded8cd0cf859ff/cmd/soroban-rpc/internal/jsonrpc.go#L139-L142

The transaction window is still different than the events window, which can be larger.