Fix band example after CP2K FFT keyword removal

[DCM-Uni-Paderborn]

The CP2K dashboard's AiiDA test fails in example_bands with KeyError: 'output_bands'. The underlying calculation aborts because CP2K removed GLOBAL/EXTENDED_FFT_LENGTHS in cp2k/cp2k#5868.

Remove the obsolete keyword and its otherwise empty GLOBAL block from the example. No calculation parameters, reference values, tolerances, or result checks are otherwise changed.

Validation:

• Reproduced the original unknown-keyword abort with CP2K master dd0921c.
• Ran the corrected example through AiiDA and its advanced parser: exit status 0; all existing k-point, band-shape, and eigenvalue checks pass.
• Repeated the corrected example with the pre-removal CP2K binary from the August 11 Spack image: all existing checks pass.
• The printed Gamma-point eigenvalues match the existing references in both builds.
• All applicable pre-commit checks pass.

Validation covers the affected end-to-end example, not the full AiiDA test suite.

CI maintenance (follow-up commit 901ac59):

• Replace retired actions/cache@v2 with v6; the plugin jobs previously failed during setup, before any tests ran.
• Update pyupgrade to v3.21.2, fixing its tokenize.cookie_re crash under Python 3.14. No hooks are skipped.
• Point Python intersphinx links at Python 3 and AiiDA API links at the published v2.7.3 reference. AiiDA removed API autodocs from newer documentation in Drop api-auto-docs for now aiida-core#7056; warnings remain fatal.

Follow-up validation:

• Reproduced all four documentation cross-reference warnings with Python 3.11, Sphinx 9.0.4 and AiiDA 2.9.2; the corrected strict make build passes and all four references produce external links.
• All pre-commit hooks pass across the repository; pyupgrade also passes under Python 3.14. The updated pre-commit.ci check is green.
• Re-ran the affected band example end-to-end: exit status 0 and all existing checks pass, with unchanged references and tolerances.
• The new GitHub Actions run is awaiting maintainer approval: https://github.com/aiidateam/aiida-cp2k/actions/runs/34052461558. The full container test matrix has therefore not yet been validated on this revision.