unmaintained/s6-overlay: move from community

Upstream development appears to have stopped several months ago. Recent s6-overlay versions do not work correctly with recent execline versions (which are separately packaged for Alpine by skarnet).

For more information see https://github.com/just-containers/s6-overlay/issues/329

Closes #12547 (closed).

If/when upstream resolves compatibility with general s6/execline packages then I will restart maintaining this package.

For now the only solution for end-users is to download and use upstream's arch-specific binary tarfiles at https://github.com/just-containers/s6-overlay/releases which contain versions of s6/execline binaries that upstream has tested with s6-overlay.