scripted-diff: Rename DEFAULT_MAX_MEMPOOL_SIZE to indicate SI unit

Summary:
Better to be explicit when it comes to sizes to avoid unintentional
bugs. We use MB and KB all over the place.

-BEGIN VERIFY SCRIPT-
find_regex="DEFAULT_MAX_MEMPOOL_SIZE" \
    && git grep -l -E "$find_regex" \
        | xargs sed -i -E "s@$find_regex@\0_MB@g"
-END VERIFY SCRIPT-

This is a partial backport of core#25290
https://github.com/bitcoin/bitcoin/pull/25290/commits/ccbaf546a68d6cda8ed3efd0598c0e4121b366bb

Depends on D15297

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15298