rand: deprecate in favor of math/rand/v2

For golang/go#61716
Fixes golang/go#71373

Change-Id: I478f963fc6ecd7de540e288ffa5f4f6dd87219d7
Reviewed-on: https://go-review.googlesource.com/c/exp/+/644475
Reviewed-by: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

Author: ianlancetaylor

rand/rand.go

@@ -4,17 +4,11 @@
 
 // Package rand implements pseudo-random number generators.
 //
-// Random numbers are generated by a Source. Top-level functions, such as
-// Float64 and Int, use a default shared Source that produces a deterministic
-// sequence of values each time a program is run. Use the Seed function to
-// initialize the default Source if different behavior is required for each run.
-// The default Source, a LockedSource, is safe for concurrent use by multiple
-// goroutines, but Sources created by NewSource are not. However, Sources are small
-// and it is reasonable to have a separate Source for each goroutine, seeded
-// differently, to avoid locking.
-//
 // For random numbers suitable for security-sensitive work, see the crypto/rand
 // package.
+//
+// Deprecated: use the math/rand/v2 package instead. This package is
+// scheduled to be tagged and deleted, per https://go.dev/issue/61716.
 package rand
 
 import "sync"