multifilesrc: fix caps leak

since `gst_caps_replace()` and `gst_pad_set_caps()` both ref the caps and neither of them takes the ownership of the caps -> it must be unreffed in `gst_multi_file_src_set_property()`

to test the leak (on Unix): `echo coucou > /tmp/file.txt && GST_TRACERS=leaks GST_DEBUG="GST_TRACER:7" gst-launch-1.0 multifilesrc location=/tmp/file.txt caps='txt' ! fakesink`

Part-of: <gstreamer/gst-plugins-good!1109>
32 jobs for !1109 with backport-mr-1436-into-1.18-multifilesrc_caps_leak in 39 minutes and 42 seconds (queued for 2 seconds)
merge request