Fix GitHub action that submits the updated rendered files when a PR is merged to main

[climbfuji]

See https://github.com/ESCOMP/ESMStandardNames/actions/runs/16168953364/job/45637458710 and #117 (comment)

[mkavulich]

We decided via discussion on #122 to revert to the old way of updating the human-readable standard names files, but with better documentation/instructions