fix: checks out selected branch instead of main in db update workflow

[jcpitre]

Closes #1510

The gist of this PR is that the checkout in db-update.yml is now using the branch that's associated with the run instead of main.
This was OK for releases (since the associated branch is usually main) but not for manual trigger, where the user can select another branch.

Also added a README.md for the workflows, partly generated with AI, but also somewhat vetted by a human.

Also removed db-update-schema.yml, that is currently unused (it can only be called from another workflow, and nobody is calling it), AND does the same job as db-update.yml.

Also I corrected some minor typos related to some license examples in DatabaseCatalogAPI and OperationsAPI.yaml. Replaced OBSD with 0BSD (with a zero)

Testing tips:

Ran db-update-dev.yml manually with a certain branch and DRY_RUN and made sure that git got the right branch (it's in the logs).

Checking out the ref
  /usr/bin/git checkout --progress --force -B 1510-db-update-workflow-checks-out-main-instead-of-the-selected-branch refs/remotes/origin/1510-db-update-workflow-checks-out-main-instead-of-the-selected-branch

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[github-actions]

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[qcdyx]

I tested it on QA, it ran on the selected branch, but failed at liquibase installation.

[jcpitre]

I tested it on QA, it ran on the selected branch, but failed at liquibase installation.

I suspect liquibase download is flaky. It works ok if we rerun the GH action.