Backport of scaling: fix state store corruption bug for job scaling events into release/1.8.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #23673 to be assessed for backporting due to the inclusion of the label backport/1.8.x.

The below text is copied from the body of the original PR.

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When updating a JobScalingEvent, the state store function did not copy the existing object before mutating it. This corrupts the state store because it modifies the leaf node without committing it in a transaction. It can also cause the Nomad server to crash with a "fatal error: concurrent map read and map write" if its ScalingEvents map is read via the ScaleStatus RPC at the same time as it's being written.

This changeset also removes some mostly-unused public methods on the struct that dangerously encourage you to mutate it outside of a copy.

Ref: https://hashicorp.atlassian.net/browse/NET-10529

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Overview of commits

• 92d216f

[github-actions]

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