Update backport workflow to use default GITHUB_TOKEN

[jdbaldry]

Use the default token injected into workflows since the GH_BOT_ACCESS_TOKEN is revoked.

The default token doesn't have the permission used for unassigning default reviewers. If you don't have it, you get cryptic comments posted back on the PR after the backport PRs are opened with the message Validation Failed: "Could not resolve to a node with the global id of '<NODE ID>."

Like in grafana/pyroscope#3557 (comment)

Behavior was documented in grafana/grafana-github-actions#226 and made configurable in grafana/grafana-github-actions#227

removeDefaultReviewers: false instructs the workflow to not try to avoid the error but will of course not remove the default reviewers assigned on the backport PR.

The default token also cannot trigger CI so required checks will have to manually triggered with a commit not from this token. There are plans to provide this behavior as a shared workflow in https://github.com/grafana/shared-workflows but it may be some weeks before this is available.

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[ptodev]

It would be nice to refer to a version, so that we have to update the workflow explicitly when new functionality is available? I suppose that now it'll just pull the latest from main?