Context

This MR one part of a split from !4580 (closed). It extracts the protocol's parametric constants into their own files with two purposes:

Make distinction between current and previous constants more clear
Enable scripting the creation of the files that represent the previous protocol parameters (this is the second part of the split)

Manually testing the MR

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
Select suitable reviewers using the Reviewers field below.
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