Deprecate linearindices in favor of LinearIndices (#26775)

* Deprecate linearindices in favor of LinearIndices

LinearIndices is strictly more powerful than linearindices: these two functions
return arrays holding the same elements, but the former also preserves the shape
and indices of the original array.

Also improve docstrings.

* Add efficient LinearIndices iteration

* Work around invalidation and minor LinearIndices simplifications

* Alternative fix using eachindex + adjust failing test

* Fix accumulate/cumsum performance regression by adding getindex(::LinearIndices, ::AbstractRange)

Plus two small fixes.

base/statistics.jl

@@ -137,7 +137,7 @@ function centralize_sumabs2!(R::AbstractArray{S}, A::AbstractArray, means::Abstr
 
     if has_fast_linear_indexing(A) && lsiz > 16
         nslices = div(_length(A), lsiz)
-        ibase = first(linearindices(A))-1
+        ibase = first(LinearIndices(A))-1
         for i = 1:nslices
             @inbounds R[i] = centralize_sumabs2(A, means[i], ibase+1, ibase+lsiz)
             ibase += lsiz