qt: increase console command max length

Summary:
This fixes an issue of json/hex inputs being truncated for very long transactions. Fix core#17618

Rationale for the value:

The largest p2p message type is a 4 MB block? With hex encoding that should be 8 MB. Wrapped again in json, that should be less than 16 MB.

This is a backport of core#18993

Test Plan: Use decoderawtransaction with a very large transaction (e.g. b58309875b2d13d90531f8f5fd54705b6439a49957ad23727e506fbefa51913f) in the GUI console.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9986