QUICDatagramsSent or UDPDatagramsSent

[rmarx]

We currently have several events that can be used to indicate how QUIC packets are sent in UDP datagrams (quic:datagrams_sent, quic:datagrams_received etc.).

However, adding support for RFC9221 (Unreliable Datagram extension for QUIC), which adds the new DatagramFrame, causes some issues with the naming here.

For example, we also want a datagram_data_moved event in addition to the existing (stream_)data_moved event, but that would be ambiguous which "datagram" we're actually talking about (UDP datagrams or QUIC Datagram Frames).

Since we've renamed the transport category to quic (before it was transport:datagrams_*), it makes sense to rename these events to udp:datagrams_* as well for consistency and clarity.