File the inequality change under the release that contains it

[Rafael-SOWNet]

BREAKING-CHANGES.md records the polynomial-inequality change under ## 2.3.0 — since 2.2.0.
It merged today as #1017, after v2.3.0 was tagged:

$ git show v2.3.0:BREAKING-CHANGES.md | grep -c "polynomial inequality of degree three"
0

So the file tells a reader that "x ^ 3 - x > 0".Solve("x") changed in 2.3.0, and anyone who
installs 2.3.0 to check will find it refusing exactly as before. Recording which release an answer
moved in is the one thing this file exists to get right — its own preamble says every entry gives
"the value on the previous release and the value now, both measured on a build".

What changed

The at-a-glance row and the ### A polynomial inequality of degree three or more is answered section
move verbatim from ## 2.3.0 to ## Unreleased — since 2.3.0. No wording changes.

Verified rather than eyeballed — sorting the whole file before and after gives an identical multiset
of lines, so nothing was added, reworded or lost in the move:

$ diff <(git show origin/master:BREAKING-CHANGES.md | sort) <(sort BREAKING-CHANGES.md)
$ echo $?
0

Merge order

BREAKING-CHANGES.md is also edited by #1025 and #1033, both of which add to ## Unreleased.
Whichever merges later pays one mechanical conflict round; this one only moves an existing block, so
resolving it is keeping both sides.