doc: Better instructions for single-dependency upgrade/downgrade (#27516)
doc: Better instructions for single-dependency upgrade/downgrade (#27516)

This change embraces persistent min-version dependencies, which can
simply be ratcheted up over time, or safely removed in time because IDAs
rarely have a large dependency downgrade.

Some dependencies are already encoded with a min-version constraint, which
makes the previous instructions confusing. There was also a very minor
issue in which the temporary constraint line was adding a spurious
`-c constraints.txt` to the committed changes, even though the constraint
itself was not being committed.

This change inspired by discussion on PR #27506.
arbrandes/edx-platform,eduNEXT/edx-platform,eduNEXT/edx-platform,edx/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,edx/edx-platform,angelapper/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,angelapper/edx-platform,edx/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,edx/edx-platform