Add compat note in Dates.periods docstring (#39189)

Author: sostock

stdlib/Dates/src/periods.jl

@@ -200,6 +200,9 @@ end
     Dates.periods(::CompoundPeriod) -> Vector{Period}
 
 Return the `Vector` of `Period`s that comprise the given `CompoundPeriod`.
+
+!!! compat "Julia 1.7"
+    This function requires Julia 1.7 or later.
 """
 periods(x::CompoundPeriod) = x.periods