Use Gst.Element.request_pad_simple instead of Gst.Element.get_request_pad

[ryonakano]

../src/Services/Recorder.vala:144.56-144.76: warning: `Gst.Element.get_request_pad' has been deprecated since 1.20
  144 |                 mic_sound.get_static_pad ("src").link (mixer.get_request_pad ("sink_%u"));
      |                                                        ^~~~~~~~~~~~~~~~~~~~~              
../src/Services/Recorder.vala:145.56-145.76: warning: `Gst.Element.get_request_pad' has been deprecated since 1.20
  145 |                 sys_sound.get_static_pad ("src").link (mixer.get_request_pad ("sink_%u"));
      |                                                        ^~~~~~~~~~~~~~~~~~~~~              
../src/Services/Recorder.vala:154.50-154.70: warning: `Gst.Element.get_request_pad' has been deprecated since 1.20
  154 |             encoder.get_static_pad ("src").link (muxer.get_request_pad ("audio_%u"));
      |                                                  ^~~~~~~~~~~~~~~~~~~~~               
Compilation succeeded - 3 warning(s)

Gst.Element.request_pad_simple is available on 1.20 or later which is available in the jammy so marking as blocked until that version gets available in the elementary SDK