Update gstreamer-rtsp-server to 1.20.1 to match the version submitted to SP4

- Remove BuildRequires: hotdoc and disable the doc generation.
It's really not used at all.

- Update to version 1.20.1:
+ Fix race in rtsp-client when tunneling over HTTP

- Update to version 1.20.0:
+ GstRTSPMediaFactory gained API to disable RTCP
(gst_rtsp_media_factory_set_enable_rtcp(), "enable-rtcp"
property). Previously RTCP was always allowed for all RTSP
medias. With this change it is possible to disable RTCP
completely, irrespective of whether the client wants to do RTCP
or not.
+ Make a mount point of / work correctly. While not allowed by
the RTSP 2 spec, the RTSP 1 spec is silent on this and it is
used in the wild. It is now possible to use / as a mount path
in gst-rtsp-server, e.g. rtsp://example.com/ would work with
this now. Note that query/fragment parts of the URI are not
necessarily correctly handled, and behaviour will differ
between various client/server implementations; so use it if you
must but don't bug us if it doesn't work with third party
clients as you'd hoped.
+ multithreading fixes (races, refcounting issues, deadlocks).
+ ONVIF audio backchannel fixes.
+ ONVIF trick mode optimisations.
+ rtspclientsink: new "update-sdp" signal that allows updating
the SDP before sending it to the server via ANNOUNCE. This can
be used to add additional metadata to the SDP, for example. The
order and number of medias must not be changed, however.