RPC: Remove final comma for last entry of fixed-size Arrays and Objects in RPCResult

Summary:
JSON doesn't allow a trailing comma in Arrays/Objects

This is a backport of Core PR18444

Test Plan:
Look at the help message for a command that return a JSON object, and verify that the last item in the output example does not have a trailing comma

src/bitcoin-cli help getblockstats

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8892