Make dask[distributed] optional via covalent[dask]- no dask install by default

[araghukas]

Require pip install covalent[dask] to install dask[distriburted] dependency for covalent, do not install dask[distributed] otherwise, with fallback to "local" executor.

Changes summary

Main point

• Make dask[distributed] and optional dependency via covalent[dask] extra
• Default executor changed from dask to local in defaults.py

Necessary fixes

• Dropped Python 3.10 support, added Python 3.13; updated test_matrix.json and CI workflows accordingly
• Fixed extract_graph to handle both "edges" (networkx ≥3.x) and "links" (older networkx) as the edge key; added lower bound on networkx version
• Fixed various 422 errors by setting additional Content-Type: application/json headers as necessary
• Moved locust out of tests/requirements.txt into the benchmark workflow only, to avoid a gevent conflict
• Various test fixes: renamed status → job_status, corrected mock paths, updated graph key references

• I have added the tests to cover my changes.
• I have updated the documentation and CHANGELOG accordingly.
• I have read the CONTRIBUTING document.

[araghukas]

closing to try to bypass now-removed checks on Python 3.10

[araghukas]

replaced with: #2016