ci: move workflows off the retired ubuntu-20.04 runner

[k-yoshimi]

Summary

GitHub has retired the ubuntu-20.04 runner, so every workflow job in this repository stayed queued until it was cancelled after 24 hours. No tests, documentation builds, documentation deployments or PyPI releases have been running.

• Workflows: run on ubuntu-22.04, as ODAT-SE does; test with Python 3.9 and 3.12 (Python 3.6 is not available on current runners); update actions/checkout, actions/setup-python and rlespinasse/github-slug-action to the same pinned versions ODAT-SE uses. The slug action v5.6.0 still provides GITHUB_REF_SLUG, which deploy_docs.yml uses.
• exchange algorithm fix: with Python 3.12 the tests installed NumPy 2.5, and tests/exchange failed with ValueError: setting an array element with a sequence at ibuf[0] = self.Tindex. Tindex is a one-element array, and recent NumPy no longer converts it to a scalar implicitly; the failing process left the others blocked in MPI until the job timed out. It now sends Tindex[0], the same as ODAT-SE. The values sent over MPI are unchanged.

Verification

• On this branch, all 14 test jobs (7 samples × Python 3.9 / 3.12) pass, and the documentation build passes.
• Before the exchange fix, exchange on Python 3.12 timed out while the other 13 jobs passed.
• deploy_docs.yml and deploy_pypi.yml only run on pushes to develop/master and on tags, so they are not exercised by this branch; their changes are limited to the runner and action versions.

Note

pyproject.toml still declares python = ">=3.6.8", but Python 3.6 is no longer tested.

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