swapserverrpc: bump Go to 1.18

Protobuf v1.34.2 generates Go files that fail to compile with Go 1.16:
predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)

Also ran "go mod tidy".

Author: starius

swapserverrpc/go.mod

@@ -5,4 +5,12 @@ require (
 	google.golang.org/protobuf v1.34.2
 )
 
-go 1.16
+require (
+	github.com/golang/protobuf v1.5.3 // indirect
+	golang.org/x/net v0.9.0 // indirect
+	golang.org/x/sys v0.7.0 // indirect
+	golang.org/x/text v0.9.0 // indirect
+	google.golang.org/genproto v0.0.0-20230410155749-daa745c078e1 // indirect
+)
+
+go 1.18