Unversioned component URLs

Bare /component/ URLs still 404 on numbered-only Antora components. The stopgap extension now has a name that matches the choice:

unversioned-component-urls — default mode: selector (version chooser), optional mode: alias for redirect-to-latest.

Formerly alias-component-to-latest. GitHub redirects the old repo URL; update playbook require / dependency keys to @antora-supplemental/unversioned-component-urls.

Related: antora/antora#291 · Antora use case