Fix the ReadTheDocs build

[jpivarski]

The last successful build was on September 25, 2023. It looks like v5.0.12 was the latest (already existing) git tag at that time.

The very first error after that is:

The configuration key "build.os" is required to build your documentation. Read more at https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os

It looks like we just need to add

build:
  os: ubuntu-22.04

to

uproot5/.readthedocs.yml

Lines 1 to 11 in 1f69682

	version: 2
	sphinx:
	configuration: docs-sphinx/conf.py
	python:
	version: 3.7
	install:
	- requirements: docs-sphinx/requirements.txt
	- method: pip
	path: .

It's probably very easy! But this failure in documentation-building should have become an alert somehow. I'll look into how to make that happen.