[RPC] Fix casing in getblockchaininfo to be inline with the rest of the response

Summary:
The original PR changed the case of a parameter return by getblockchaininfo: startTime -> start_time
Luckily in our codebase it is only the documentation that was wrong, so this does not break compatibility. The parameter was reintroduced in Bitcoin ABC earlier this year while restoring BIP9 RPC support (D5135)

Backport of Core PR16817

Test Plan: bitcoin-cli help getblockchaininfo

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8075