Maybe that file somehow didn't get copied over into your [dspace]/config/ folder (where DSpace 7.x is installed)? Or, it's possible that your "dspace.cfg" is missing the new "include" statement which imports the "external-providers.cfg". See this new line:
https://github.com/DSpace/DSpace/blob/main/dspace/config/dspace.cfg#L1684
It's unclear which step of the "ant update" script resulted in this error... but, essentially, it sounds like it isn't finding that "external-providers.cfg", which results in the error you are experiencing.
Hopefully these hints might help give you clues on how to proceed. But, if you need more help, let us know on this list.