fix convolution constraint

[yosupo06]

Current impelmentation can convolute small arrays (min(n, m) <= 60) for arbitrary mod, e.g. $10^9 + 7$.

I started to think this behavior is weird and may confuse participants by the following reasons, so let's fix.

• "Code that only works for small data" is very dangerous in competitive programing, e.g. : uses convolution with $10^9 + 7$ -> works for samples -> submit -> got mysterious RE
• 60 is just a magic number for the constant optimization. We may change this value in future.