CompatHelper: bump compat for AbstractTrees to 0.4, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the AbstractTrees package from 0.3 to 0.3, 0.4. This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[mortenpi]

@oscardssmith What would your recommended compat for AbstractTrees be? Would it make sense / be necessary to drop 0.3?

[oscardssmith]

I think supporting both should be fairly easy. they're are a bunch of breaking changes, but none of them affect your usage.

[mortenpi]

Cool, thank you @oscardssmith!