rtpgstdepay: Only store the current caps and drop old caps immediately

rtpgstdepay: Only store the current caps and drop old caps immediately

Otherwise it can happen that we already collected 7 caps, miss the 8th
caps packet (packet loss) and then re-use the 1st caps for the following
buffers instead of the 8th caps which will likely cause errors further
downstream unless both caps are accidentally the same.

Keeping old caps around does not seem to have any value other than
potentially causing errors. We would always receive new caps whenever
they change (even if they were previous ones) and it's very unlikely
that they happen to be exactly the same as the previous ones.

Also after having received new caps or a buffer with a next caps
version, no buffers with old caps version will arrive anymore.