Use new where syntax for a couple of misc methods (#21415)

* Use new where syntax for eltype

* New where syntax for some size methods

* New where syntax for copy mehods

Author: musm

src/AbstractFFTs.jl

@@ -8,7 +8,7 @@ abstract type Plan{T} end
 import Base: show, summary, size, ndims, length, eltype,
              *, A_mul_B!, inv, \, A_ldiv_B!
 
-eltype{T}(::Type{Plan{T}}) = T
+eltype(::Type{Plan{T}}) where {T} = T
 
 # size(p) should return the size of the input array for p
 size(p::Plan, d) = size(p)[d]