crypto/rsa: deprecate multiprime RSA support

Fixes #56921

Change-Id: I03f9969a5146ab7becd983784d8cb5b23a3fbb18
Reviewed-on: https://go-review.googlesource.com/c/go/+/459976
TryBot-Bypass: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

api/next/56921.txt

@@ -0,0 +1,2 @@
+pkg crypto/rsa, func GenerateMultiPrimeKey //deprecated #56921
+pkg crypto/rsa, type PrecomputedValues struct, CRTValues //deprecated #56921

src/crypto/rsa/rsa.go

@@ -204,7 +204,7 @@ type PrecomputedValues struct {
 	// differently in PKCS #1 and interoperability is sufficiently
 	// important that we mirror this.
 	//
-	// Note: these values are still filled in by Precompute for
+	// Deprecated: These values are still filled in by Precompute for
 	// backwards compatibility but are not used. Multi-prime RSA is very rare,
 	// and is implemented by this package without CRT optimizations to limit
 	// complexity.
@@ -278,7 +278,7 @@ func GenerateKey(random io.Reader, bits int) (*PrivateKey, error) {
 // This package does not implement CRT optimizations for multi-prime RSA, so the
 // keys with more than two primes will have worse performance.
 //
-// Note: The use of this function with a number of primes different from
+// Deprecated: The use of this function with a number of primes different from
 // two is not recommended for the above security, compatibility, and performance
 // reasons. Use GenerateKey instead.
 //