Update CI workflows: AirspeedVelocity benchmarks, fix downgrade

[Krastanov-agent]

Comment from Stefan, the human: A bunch of the CI workflows were not working reliably. Benchmarks just do not run (airspeed velocity seems to be the cleanest way to run them these days), invalidation checks were outdated and not useful, and the downgrade action is missing a config option which would lead to problems when we change the julia version we are testing against.

Below is the autogenerated description from the agent:

Summary

• Delete postbench.yml and runbenchandupload.yml, replace with a single AirspeedVelocity-based benchmark.yml workflow (matching QuantumSavory pattern)
• Add julia_version parameter to julia-downgrade-compat@v2 action in downgrade.yml

Test plan

• Verify benchmark workflow triggers correctly on PRs
• Verify downgrade CI passes with the new julia_version parameter

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.22%. Comparing base (5eaa071) to head (d44c1ca).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #490   +/-   ##
=======================================
  Coverage   97.22%   97.22%           
=======================================
  Files         122      122           
  Lines        7240     7240           
=======================================
  Hits         7039     7039           
  Misses        201      201           

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[Krastanov]

it is normal that the benchmark is not running here, as we are changing the workflow file