Normalize usage of "single sign-on" (#12256)
There are a few different casing issues and punctuation styles, this
reduces down to "single sign-on" for all of our usage.
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:books: Documentation previews :books:
- User's documentation (`docs`):
https://docs--12256.org.readthedocs.build/12256/
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- Developer's documentation (`dev`):
https://dev--12256.org.readthedocs.build/12256/
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