math/rand: deprecate Seed

Programs that call Seed and then expect a specific sequence
of results from the global random source (using functions such as Int)
can be broken when a dependency changes how much it consumes
from the global random source. To avoid such breakages, programs
that need a specific result sequence should use NewRand(NewSource(seed))
to obtain a random generator that other packages cannot access.

Fixes #56319.

Change-Id: Idac33991b719d2c71f109f51dacb3467a649e01e
Reviewed-on: https://go-review.googlesource.com/c/go/+/451375
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

Author: rsc

Diff for: src/math/rand/rand.go

@@ -315,7 +315,7 @@ var globalRand = New(new(lockedSource))
 // Alternately, set GODEBUG=randautoseed=0 in the environment
 // before making any calls to functions in this package.
 //
-// Note: Programs that call Seed and then expect a specific sequence
+// Deprecated: Programs that call Seed and then expect a specific sequence
 // of results from the global random source (using functions such as Int)
 // can be broken when a dependency changes how much it consumes
 // from the global random source. To avoid such breakages, programs