reflect: deprecate SliceHeader and StringHeader

These are replaced by unsafe.String etc, which were added in Go 1.20.
Per https://go.dev/wiki/Deprecated, we must wait until Go 1.21
to mark them deprecated.

Fixes #56906.

Change-Id: I4198c3f3456e9e2031f6c7232842e187e6448892
Reviewed-on: https://go-review.googlesource.com/c/go/+/452762
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
Run-TryBot: Russ Cox <[email protected]>

Author: rsc

api/next/56906.txt

@@ -0,0 +1,2 @@
+pkg reflect, type SliceHeader //deprecated #56906
+pkg reflect, type StringHeader //deprecated #56906

src/reflect/value.go

@@ -2743,7 +2743,7 @@ func (v Value) UnsafePointer() unsafe.Pointer {
 // it references will not be garbage collected, so programs must keep
 // a separate, correctly typed pointer to the underlying data.
 //
-// In new code, use unsafe.String or unsafe.StringData instead.
+// Deprecated: Use unsafe.String or unsafe.StringData instead.
 type StringHeader struct {
 	Data uintptr
 	Len  int
@@ -2756,7 +2756,7 @@ type StringHeader struct {
 // it references will not be garbage collected, so programs must keep
 // a separate, correctly typed pointer to the underlying data.
 //
-// In new code, use unsafe.Slice or unsafe.SliceData instead.
+// Deprecated: Use unsafe.Slice or unsafe.SliceData instead.
 type SliceHeader struct {
 	Data uintptr
 	Len  int