Backport of release: always use service user for git ops into release/1.9.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24546 to be assessed for backporting due to the inclusion of the label backport/1.9.x.

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

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We are shoring up our user and bot accesses, and this seems to be our only usage of the github-actions bot.

These were the commits that have been flagged as needing adjustment:

workflows/release.yml
145:            git commit --message "Generate files for ${{ github.event.inputs.version }} release"
212:            git commit --message 'Prepare for next release'

We may later discover other of our repos that need a similar treatment, if they've just not been run during this information-gathering time, but at present this is the only known case. Our CE->ENT merge workflow in the ENT repo is already set up properly.

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Overview of commits

• e963d55