Fix naming inconsistencies across `Computer`, `Code`, and `CalcJob` APIs

[GeigerJ2]

During the systematic review of all runtime/scheduler options in #7272, several naming inconsistencies were identified across the Computer, Code, and CalcJob layers. These have grown organically over time and should be cleaned up for a consistent API.

What	Mismatch	Notes
withmpi / with_mpi	CalcJob input spec and CodeInfo field use withmpi; Code property uses with_mpi	Most impactful — user-facing input name
mpiprocs_per_machine / default_mpiprocs_per_machine	Builder/CLI key drops the default_ prefix that the Computer API uses (get_default_mpiprocs_per_machine())	Silent mismatch, no deprecation note
input_plugin / default_calc_job_plugin	DB attribute stored as input_plugin, public property is default_calc_job_plugin	Has TODO at abstract.py:37 for v3.0
workdir / work_dir	Computer API is get_workdir(), Builder/CLI key is work_dir	Minor

withmpi → with_mpi specifically

This is the most user-facing inconsistency. Affected layers:

Layer	Current	Target
AbstractCode property	with_mpi	with_mpi (already correct)
CalcJob input spec	metadata.options.withmpi	metadata.options.with_mpi
CalcJob node attribute	withmpi	with_mpi
CodeInfo field	withmpi	with_mpi

Suggested migration pattern (applies to all of the above)

1. >v2.8: Add the new name alongside the old. Internally use the new name; emit AiidaDeprecationWarning when the old name is used. If both are set and disagree, raise ValueError. Non-breaking — existing code keeps working.
2. v3.0: Remove the old aliases.
3. Database migration: optional per case — can read both names at runtime during the deprecation period, then migrate or drop in v3.0.