Rename to "codec dictionary match" algorithm and remove index consistency claim

[jan-ivar]

Fixes #2955.

Once merged, links in the following places would need to be updated:

1. https://w3c.github.io/webrtc-extensions/#addtransceiver

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[alvestrand]

Why rename?
I can't see anything else that needs to use the name "codec match", and as pointed out in the CL, renaming makes extra work in multiple other places. I fail to see the benefit of that.

The other changes in the CL look fine to me.

[jan-ivar]

@jan-ivar to merge a PR for https://w3c.github.io/webrtc-extensions/#addtransceiver right after merging this.