ci: bump clang-format 14 → 20 and reformat

[matthargett]

Summary

Bumps the coding-guidelines formatter from clang-format-14 to clang-format-20 and reformats the tree to match. This is the clang-format half of #4983's tooling modernization; the clang-tidy bump and the runner / native-LLVM parts follow separately.

What changed

• Commit 1 (config): ci/coding_guidelines_check.py and ci/pre_commit_hook_sample pin clang-format-20; coding_guidelines.yml installs it from apt.llvm.org for the running Ubuntu release (it isn't preinstalled), so the pinned version no longer drifts with the runner image — the rot behind today's -12/-14 mix. .clang-format's version note is updated (it also fixes the clang-formt typo).
• Commit 2 (reformat): mechanical clang-format-20 -i over the coding-guidelines-checked sources, as a standalone commit for easy review.

Noise: small

Only 42 of 441 files (~209 lines) change — the code was already close to the clang-format-20 style, and .clang-format needed no option changes (zero config warnings at 20). The reformat is also version-flat: 39 files at clang-format-18, 43 at 21/22, so pinning 20 carries no penalty over the runner-preinstalled 18 while being an explicitly-pinned latest stable.

Scope

Intentionally just clang-format. The clang-tidy bump to 20 is separate: at 20 the current .clang-tidy wildcards surface ~3,300 diagnostics on the tree (53% misc-include-cleaner), which needs config curation plus incremental cleanup — a follow-up tied to #4982, not bundled here.

[matthargett]

Superseded by #4991 — switched to ubuntu-24.04's preinstalled clang-format-18 (no external download) instead of installing clang-format-20 from apt.llvm.org.