DEV: See if we can simplify auto update of astropy-iers-data #19495

This issue is only actionable after we drop support for v7.x series. When our backport branch is v8.x or later, the backport branch would have its own lowest-resolved-tree.txt by then. We can see if we can revert #19472 , i.e., stop dispatching the workflow to backport branch and only run it once or main and rely on backport bot to do the backport. However, we need to close this issue as "won't fix" if it turns out auto backport is still not possible then.

The simplest way to test this is as follows:

1. Wait for v8.0.x to get branched.
2. Wait for .github/workflows/update_astropy_iers_data_main.yml to run again.
3. Ignore the PR opened against v8.0.x
4. Merge the PR opened against main. Trigger auto-backport bot for this PR for v8.0.x instead.
   a. If successful, we can revert Change the way we update astropy-iers-data for active branches #19472 and close the PR in (3) without merge.
   b. If not successful, we need the PR in (3) after all, and close this issue as "won't fix"