proposal: math/rand: add InsecureRand

[BobDu]

#20661
now, we only deprecate math/rand.Read , in initial proposal suggest add a math/rand.InsecureRand

in many cases (like only create a random string in internal system for idempotent), we need math/rand.
So, we should give developers a choice.

[seankhliao]

Only the global function is deprecated, we're not going to add back a new global, whatever the name is.
You can create your own package level one if you need.