[CMake][Python] Bump minimum CMake version to 3.26 (#23607)

CMake 3.26 added FindPython's `Development.SABIModule` component, which
is required for building Stable ABI (abi3). With this minimum version,
we can unconditionally request `SABIModule` upfront in the Python
`find_package` calls, before deciding whether to enable abi3. This is
appealing because the SABI module must be found early and cannot be
deferred to after the abi3 option is resolved; otherwise we need the
user to decide through cmake flags.

CMake 3.26 availability:
  pip install cmake: 4.2.3
  Homebrew: 4.2.3
  Chocolatey (Windows CI): 4.2.3
  Ubuntu 24.04 (apt): 3.28.3
  Ubuntu 22.04 (apt): 3.22.1 (use pip or Kitware APT repo)
  Debian 12 bookworm (apt): 3.25.1 (3.31.6 in backports)
  
ci-extra: all

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>