chore: bump version to 21.1.0

[paulirwin]

Bumps @feature23/ngx-editor from 21.0.0 → 21.1.0 ahead of the v21.1.0 release.

Changes since v21.0.0

• fix(toolbar): keyboard focus and accessibility improvements (#586) #6 fix(toolbar): keyboard focus and accessibility improvements (upstream #586)
• fix(editor): fix undo clearing out editor after initial load (#638) (#641) #7 fix(editor): fix undo clearing out editor after initial load (upstream #641)
• fix(editor): fix horizontal rule not allowing input on empty editor (#635) (#642) #8 fix(editor): fix horizontal rule not allowing input on empty editor (upstream #642)

All three are bugfixes pulled from upstream PRs — minor version bump is appropriate.

Release steps after merge

1. git checkout master && git pull
2. gh release create v21.1.0 --title "v21.1.0" --notes "..."
3. CI (.github/workflows/npmpublish.yml) publishes to npm via OIDC