grpc: deprecate WithSharedWriteBuffer and SharedWriteBuffer (#9060)

Since #8957 shared write buffers are always enabled by default and
passing `false` is no longer beneficial. This PR deprecates both options
per the request in #8966, replacing the `# Experimental` notice with a
`Deprecated:` godoc comment consistent with the pattern used elsewhere
in `dialoptions.go` and `server.go`.

The options are left functional for now; removal can follow in a
subsequent release.

Fixes #8966

RELEASE NOTES:
* grpc: deprecate `WithSharedWriteBuffer` dial option and
`SharedWriteBuffer` server option; shared write buffers are now always
enabled.

Author: alliasgher

dialoptions.go

@@ -173,10 +173,8 @@ func newJoinDialOption(opts ...DialOption) DialOption {
 // If this option is set to true every connection will release the buffer after
 // flushing the data on the wire.
 //
-// # Experimental
-//
-// Notice: This API is EXPERIMENTAL and may be changed or removed in a
-// later release.
+// Deprecated: shared write buffer is enabled by default. WithSharedWriteBuffer
+// will be removed in a future release.
 func WithSharedWriteBuffer(val bool) DialOption {
 	return newFuncDialOption(func(o *dialOptions) {
 		o.copts.SharedWriteBuffer = val

server.go

@@ -250,10 +250,8 @@ func newJoinServerOption(opts ...ServerOption) ServerOption {
 // If this option is set to true every connection will release the buffer after
 // flushing the data on the wire.
 //
-// # Experimental
-//
-// Notice: This API is EXPERIMENTAL and may be changed or removed in a
-// later release.
+// Deprecated: shared write buffer is enabled by default. SharedWriteBuffer
+// will be removed in a future release.
 func SharedWriteBuffer(val bool) ServerOption {
 	return newFuncServerOption(func(o *serverOptions) {
 		o.sharedWriteBuffer = val