Set version to 3.1.1 (#6410)

Author: ximinez

src/libxrpl/protocol/BuildInfo.cpp

@@ -36,7 +36,7 @@ namespace BuildInfo {
 //  and follow the format described at http://semver.org/
 //------------------------------------------------------------------------------
 // clang-format off
-char const* const versionString = "3.1.1-rc1"
+char const* const versionString = "3.1.1"
 // clang-format on
 
 #if defined(DEBUG) || defined(SANITIZER)