Fix unimplemented Xoshiro RNG in 1.6

Author: MilesCranmer

test/test_optim.jl

@@ -1,10 +1,10 @@
 using DynamicExpressions, Optim, Zygote
-using Random: Xoshiro
+using Random: MersenneTwister as RNG
 
 operators = OperatorEnum(; binary_operators=(+, -, *, /), unary_operators=(exp,))
 x1, x2 = (i -> Node(Float64; feature=i)).(1:2)
 
-X = rand(Xoshiro(0), Float64, 2, 100)
+X = rand(RNG(0), Float64, 2, 100)
 y = @. exp(X[1, :] * 2.1 - 0.9) + X[2, :] * -0.9
 
 original_tree = exp(x1 * 0.8 - 0.0) + 5.2 * x2