Update next branch to reflect new release-train "v19.2.0-next.0".

[jkrems]

The previous "next" release-train has moved into the release-candidate phase. This PR updates the next branch to the subsequent release-train.

Also this PR cherry-picks the changelog for v19.1.0-rc.0 into the main branch so that the changelog is up to date.

[alan-agius4]

You need to run yarn bazel run @npm2//:sync, and commit the generated hash file.

We should do this as part of ng-dev as well.

//cc @devversion

[jkrems]

You need to run yarn bazel run @npm2//:sync, and commit the generated hash file.

We should do this as part of ng-dev as well.

//cc @devversion

Done!

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