PlugsManager: Replace deprecated Module.SUFFIX

[ryonakano]

Fixes the following build warning:

../lib/PlugsManager.vala:80.42-80.54: warning: `GLib.Module.SUFFIX' has been deprecated since 2.76
   80 |                     if (path.has_suffix (Module.SUFFIX)) {
      |                                          ^~~~~~~~~~~~~

GLib.Module.open() detects files that are not a shared library these days as written in the original C source code:

https://gitlab.gnome.org/GNOME/glib/-/blob/2.80.0/gmodule/gmodule.c?ref_type=tags#L87-108
https://gitlab.gnome.org/GNOME/glib/-/blob/2.80.0/gmodule/gmodule.c?ref_type=tags#L429-464

You can test this branch by the following steps:

user@elementary-daily:~$ cat dummy.vala
public static int main (string[] args) {
    return 0;
}
user@elementary-daily:~$ valac ./dummy.vala
user@elementary-daily:~$ sudo mv dummy /usr/lib/aarch64-linux-gnu/switchboard-3/system/libdummy.so
user@elementary-daily:~$ io.elementary.settings

(io.elementary.settings:5374): Gtk-WARNING **: 22:45:42.402: Theme parser error: Granite.css:1:1-6: Unknown name of pseudo-class

(io.elementary.settings:5374): Gtk-WARNING **: 22:45:42.431: Theme parser error: Granite-dark.css:1:1-6: Unknown name of pseudo-class

(io.elementary.settings:5374): switchboard-CRITICAL **: 22:45:42.512: PlugsManager.vala:50: /usr/lib/aarch64-linux-gnu/switchboard-3/system/libdummy.so: cannot dynamically load position-independent executable