Default branch has changed from "master" to "main"

[jmvalin]

The default branch for the Opus repository has changed from "master" to "main". It is more than merely a name change, as main points to what used to be the opus-ng branch and thus has many improvements over what was in master. For more details, see https://lists.xiph.org/pipermail/opus/2024-January/004585.html

Please report any issue you may encounter due to that change.