rpcserver: Use func for semver string.

This constructs the RPC API semantic version string from the individual
major, minor, and path constants versus requiring updates in both
places.  This is something contributors have previously forgotten to
update, so it makes sense to avoid the need to do it altogether.

internal/rpcserver/rpcserver.go

@@ -54,10 +54,9 @@ import (
 
 // API version constants
 const (
-	jsonrpcSemverString = "6.1.1"
-	jsonrpcSemverMajor  = 6
-	jsonrpcSemverMinor  = 1
-	jsonrpcSemverPatch  = 2
+	jsonrpcSemverMajor = 6
+	jsonrpcSemverMinor = 1
+	jsonrpcSemverPatch = 2
 )
 
 const (
@@ -99,6 +98,11 @@ const (
 )
 
 var (
+	// jsonrpcSemverString is the RPC server's semantic API version formatted as
+	// a string.
+	jsonrpcSemverString = fmt.Sprintf("%d.%d.%d", jsonrpcSemverMajor,
+		jsonrpcSemverMinor, jsonrpcSemverPatch)
+
 	// blake256Pad is the extra blake256 internal padding needed for the
 	// data of the getwork RPC.  It is set in the init routine since it is
 	// based on the size of the block header and requires a bit of