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
just-containers/s6-overlay#329

Closes #12547.

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.