Cut 21.0.0 and bump the package version in pyproject.toml, CITATION.cff,
and uv.lock.

Major: two breaking changes ship since 20.0.0. Derived-path builds emit
section source record URLs on the upstream repository supporting sources
entry (infores:pubmed-central for PMC sections) instead of the primary
graph-infores entry, and the inlined supporting Study is named after the
table config file plus its 1-based section index (#220); validate-infores
is folded into validate-kgx as the knowledge-source identity half, and
the standalone validate-infores command is gone (#218). Also adds no_map
on NodeEncoding to pass subject/object CURIEs through verbatim (#219) and
normalizes documentation prose to plain ASCII (#217).

Changelog:
- Versioned the Unreleased section as 21.0.0 - 2026-10-15.
- Added the missing #218 (breaking, with migration note), #219, and #217
  entries so no merged PR ships unmentioned.

Docs: none needed here; the release commit only versions existing prose.

Testing:
- make check -> ruff clean, pyright 0 errors, pytest 1239 passed + 1
  skipped, cargo test ok, cargo clippy --all-targets -- -D warnings clean
- uv run mkdocs build --strict -> pass