update_authors: only add names to AUTHORS file

Description

Evidently the current AUTHORS generation doesn't work with squashing commits during merge, as that could remove author names from the Git history (if those authors have made no other commits). As a fix, make update-authors now only adds names to AUTHORS files, but never removes them. The idea is that author names get added during development or when submitting the MR (in order for lint:authors to pass), but then remain on the author list even if the MR commits are squashed.

As a result the .historical_authors files are no longer needed; all author names (regardless of whether they came from Git, CVS, copyright, etc.) are just added to AUTHORS.

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

Please provide details on any reviews related to this change and and the associated reviewers.