Updating `image-labler` GitHub Action

[dysbulic]

The task that runs when a new issue is opened, issue-labler, is powered by an Action that's three years out of date & relies on Node v12. GitHub Actions no longer supports Node v12, forces the Action to run on v16.

The actual error that is causing the task to fail though is attempting an HTTP DELETE on https://api.github.com/repos/dendronhq/dendron/issues/3950/labels/scope.schema which returns HTTP 404 where #3950 was the issue I created, and it never had the scope.schema tag, nor should it have had.

All this code does is upgrade the version of issue-labler & changes how the GitHub token is accessed to match the documentation. My hopes are that the new plugin will handle deleting a non-existent tag more gracefully. (That & not want to run on Node v12.)

I can't really test this without deploying it. In fact it has to be in master for the issue-labler to run, so I should set this up on a separate repo & see if it works…

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