tree-wide: replace obsolete wiki links with systemd.io/manpages

tree-wide: replace obsolete wiki links with systemd.io/manpages

All wiki pages that contain a deprecation banner
pointing to systemd.io or manpages are updated to
point to their replacements directly.

Helpful command for identification of available links:
git grep freedesktop.org/wiki | \
    sed "s#.*\(https://www.freedesktop.org/wiki[^ $<'\\\")]*\)\(.*\)#\\1#" | \
    sort | uniq