Document that Length parameter needs Julia 1.7 (JuliaLang#45961)

* Document that `L`ength parameter needs Julia 1.7

vtjnash added the 4th type parameter `L` in JuliaLang#41619 to `StepRangeLen`.
That addition seems to have appeared in Julia 1.7 so it should be documented as such.
I found this out the hard way because my CI for code that used `L` is failing with Julia 1.6.

Co-authored-by: Johnny Chen <[email protected]>
Co-authored-by: Keno Fischer <[email protected]>

Author: 3 people

base/range.jl

@@ -467,6 +467,9 @@ value `r[1]`, but alternatively you can supply it as the value of
 `r[offset]` for some other index `1 <= offset <= len`. In conjunction
 with `TwicePrecision` this can be used to implement ranges that are
 free of roundoff error.
+
+!!! compat "Julia 1.7"
+    The 4th type parameter `L` requires at least Julia 1.7.
 """
 struct StepRangeLen{T,R,S,L<:Integer} <: AbstractRange{T}
     ref::R       # reference value (might be smallest-magnitude value in the range)