fix(deps): bump @commitlint packages to v20

[jhoblitt]

What

Bump all @commitlint/* dependencies from v19 to v20 (and
commitlint-plugin-function-rules to 4.3.2 for peer compatibility), regenerating
package-lock.json.

Why

commitlint 19 bundles conventional-commits-parser@5.x, which misparses a commit
body line that begins with an issue reference (e.g. #123) as the start of the
footer. With footer-leading-blank enabled that yields a false positive, because
the "footer" it invented is not preceded by a blank line.

Minimal repro (fails on the action as shipped, wagoid/commitlint-github-action@v6.2.1):

fix: don't frobnicate the foobar

This commit prevents the foobar from being frobnicated
during operation, and therefore fixes #69.

✖   footer must have leading blank line [footer-leading-blank]

commitlint 20 upgrades the parser to conventional-commits-parser@6.x, which parses
this correctly. Verified locally: the same message fails under the bundled 19.8.1 and
passes under 20.5.3.

Upstream tracker: #896 (open since 2020); see also the reproduction in
conventional-changelog/commitlint#4099.

Why v20 and not v21

commitlint-plugin-function-rules@4.3.x declares peerDependencies["@commitlint/lint"] = ">=19 <21", so it does not yet support commitlint 21 (that needs the plugin's v5
line, which in turn requires commitlint 21 and drops <21). v20 is the highest release
compatible with the currently-pinned plugin.

Node

No Node change required. commitlint 20 only needs Node 18+; the repo's pinned
20.16.0 (Dockerfile / .nvmrc / engines) already satisfies it.

Verification

• npm run build — ok
• npm test — 39/39 pass
• npm run lint (eslint) — clean
• Self-lint of this repo's config under commitlint 20 — clean
• The repro message above now passes; the parser is conventional-commits-parser@6.3.0
  under @commitlint/parse@20.5.0