policy-node: accept TRUE as a valid boolean property value

when coming from a gstreamer element's stream-properties property,
the boolean values use capitals:

 gst-launch-1.0 ... ! pipewiresink stream-properties="p,foo=true"
 pw-cli info <node>
   -> "foo" = "TRUE"
6 jobs for passthrough in 1 minute and 26 seconds (queued for 14 seconds)
Status Name Job ID Coverage
  Container
passed container_fedora #14677856

00:00:24

passed container_ubuntu #14677857

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  Build
passed build_on_fedora_no_docs #14677859

00:00:45

passed build_on_fedora_with_docs #14677858

00:00:53

passed build_on_ubuntu_no_gir #14677861

00:01:01

passed build_on_ubuntu_with_gir #14677860

00:00:58