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 <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Russ Cox <rsc@golang.org>

api/next/56906.txt

diff --git a/api/next/56906.txt b/api/next/56906.txt
new file mode 100644
index 0000000..aa4424a
--- /dev/null
+++ b/api/next/56906.txt
@@ -0,0 +1,2 @@
+pkg reflect, type SliceHeader //deprecated #56906
+pkg reflect, type StringHeader //deprecated #56906

src/reflect/value.go

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 5feca61..22f3eb0 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -2743,7 +2743,7 @@
 // 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 @@
 // 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