Post-transition cleanup: improvements after DIRAC is retired

[aldbr]

User Story

As a diracx developer, I want to track all the improvements that are deferred until DIRAC is no longer in use,
So that we can clean up legacy conventions once backward compatibility is no longer required.

Feature Description

Several improvements in diracx are currently deferred because they would break
backward compatibility with DIRAC (shared databases, shared config, etc.).

This issue serves as a blocker for all such improvements. Once the transition
from DIRAC to diracx is complete, the issues blocked by this one become
actionable.

Definition of Done

• DIRAC is no longer in production use