Use constants for the coinbase reserved size in the block assembler

Summary:
This avoids magic constants and makes the code self-documenting.
There is no change in behavior.

The use of std::clamp instead of min and max makes the code more readable.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17791