ws: Replace deprecated PermissionsStartOnly systemd option
ws: Replace deprecated PermissionsStartOnly systemd option

systemd 231 replaced it with the `+` prefix, and 240 deprecated it. All
our supported OSes have newer systemd versions now, and our ./configure
even demands ≥ 235.

Closes #14207
deryni/cockpit,garrett/cockpit,mvollmer/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,deryni/cockpit,garrett/cockpit,deryni/cockpit,moraleslazaro/cockpit,garrett/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit