Rename RTCEncoderIdentifier to VideoEncoderIdentifier
<rdar://problem/100402861>
Pull request: https://github.com/WebKit/WebKit/pull/4701
Committed 254903@main (d6b7b72eeeda): <https://commits.webkit.org/254903@main> Reviewed commits have been landed. Closing PR #4701 and removing active labels.