Add upgrading notes for the new Events RPC interface. (#7993)

Add deprecation logs when websocket is enabled
As promised in ADR 075, this causes the node to log (without error) when
websocket transport is enabled, and also when subscribers connect.

internal/rpc/core/env.go

@@ -287,6 +287,8 @@ func (env *Environment) StartService(ctx context.Context, conf *config.Config) (
 		if conf.RPC.ExperimentalDisableWebsocket {
 			rpcLogger.Info("Disabling websocket endpoints (experimental-disable-websocket=true)")
 		} else {
+			rpcLogger.Info("WARNING: Websocket RPC access is deprecated and will be removed " +
+				"in Tendermint v0.37. See https://tinyurl.com/adr075 for more information.")
 			wmLogger := rpcLogger.With("protocol", "websocket")
 			wm := rpcserver.NewWebsocketManager(wmLogger, routes,
 				rpcserver.OnDisconnect(func(remoteAddr string) {

internal/rpc/core/events.go

@@ -38,6 +38,8 @@ func (env *Environment) Subscribe(ctx context.Context, query string) (*coretypes
 		return nil, errors.New("maximum query length exceeded")
 	}
 
+	env.Logger.Info("WARNING: Websocket subscriptions are deprecated and will be removed " +
+		"in Tendermint v0.37. See https://tinyurl.com/adr075 for more information.")
 	env.Logger.Info("Subscribe to query", "remote", addr, "query", query)
 
 	q, err := tmquery.New(query)