Rename "next" prerelease channel to "canary" (#26761)

Summary:
The "next" prerelease channel represents what will be published the next
time we do a stable release. We publish a new "next" release every day
using a timed CI workflow.

When we introduced this prerelease channel a few years ago, another name
we considered was "canary". But I proposed "next" instead to create a
greater distinction between this channel and the "experimental" channel
(which is published at the same cadence, but includes extra experimental
features), because some other projects use "canary" to refer to releases
that are more unstable than how we would use it.

The main downside of "next" is someone might mistakenly assume the name
refers to Next.js. We were aware of this risk at the time but didn't
think it would be an issue in practice.

However, colloquially, we've ended up referring to this as the "canary"
channel anyway to avoid precisely that confusion.

So after further discussion, we've agreed to rename to "canary".

This affects the label used in the version string (e.g.
`18.3.0-next-a1c2d3e4` becomes `18.3.0-canary-a1c2d3e4`) as well as the
npm dist tags used to publish the releases. For now, I've chosen to
publish the canaries using both `canary` and `next` dist tags, so that
downstream consumers who might depend on `next` have time to adjust. We
can remove that later after the change has been communicated.

DiffTrain build for commit https://github.com/facebook/react/commit/2c2476834a7c739ca239750b73f74aa1df144a93.

Changelog: [Internal]:
<< DO NOT EDIT BELOW THIS LINE >>

Reviewed By: sammy-SC

Differential Revision: D45531681

Pulled By: tyao1

fbshipit-source-id: 27ebe14eabad5c92fe6e5aa4944ff798b808ec33

packages/react-native/Libraries/Renderer/REVISION

@@ -1 +1 @@
-fa4314841e7fdeef6e6bc5a7722fe74dc8e9fd89
+2c2476834a7c739ca239750b73f74aa1df144a93