[Refactor] Rename nLockTimeCutoff in ContextualCheckTransaction to nMedianTimePast

Summary:
For restricting nVersion based on MTP, we need MTP to be in ContextualCheckTransaction. Luckily, there's already nLockTimeCutoff, which is currently always set to the MTP.

This diff renames that parameter to nMedianTimePast so it's clear this is always the case.

Test Plan: CI should catch this

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13155