Adjust IERS update workflow to open PRs against v5.0.x and v5.3.x - #15013

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Cron jobs can only run on main - but we do check out the correct branches

Okay, in that case, maybe a follow-up issue milestoned to 6.0 so we remember to disable it when LTS is done. Thanks!

Issue opened at #15030

I won't merge yet since you need that other thing to get in first.

There is a conflict. Please rebase, then we can merge. Thanks!

@pllim - rebased!