Move the deprecation of midpoints to 0.6 section, (#20058)

and actually deprecate it - this was moved to deprecated.jl
in #16450, but not actually deprecated

Author: tkelman

docs/src/special.md

@@ -232,7 +232,6 @@ Base.varm
 Base.middle
 Base.median
 Base.median!
-Base.midpoints
 Base.quantile
 Base.quantile!
 Base.cov