all: scrub stale gRPC references from comments and docs

Reword comments and docs that still named the removed gRPC stack, rename
the GRPCSortable test interface, and drop the dead gRPC port note from the
dev README.

integration/README.md

@@ -159,7 +159,7 @@ for _, client := range allClients {
 
 Typical blocking operations: any `tailscale set` (routes, exit node,
 accept-routes, ssh), node registration via the CLI, user creation via
-gRPC.
+the CLI.
 
 ### The four rules