Update maven badge URL

Since 2026 maven-badges.herokuapp.com has been migrated to
maven-badges.sml.io.

Update our README accordingly.

For more info see:
https://github.com/softwaremill/maven-badges/blob/master/readme.md

Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
(cherry picked from commit a6200ce)

Author: ihrasko

README.md

@@ -5,7 +5,7 @@ Software-Defined Networking (SDN) solutions in Java. Developed by [PANTHEON.tech
 It utilizes core [OpenDaylight](https://www.opendaylight.org/) components, which are available as a set of libraries and are adapted to run in a __plain Java SE environment__.
 
 [![Build Status](https://github.com/PANTHEONtech/lighty/workflows/Build/badge.svg?branch=master)](https://github.com/PANTHEONtech/lighty/actions)
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.lighty.core/lighty-bom/badge.svg)](https://maven-badges.herokuapp.com/maven-central/io.lighty.core/lighty-bom)
+[![Maven Central](https://maven-badges.sml.io/maven-central/io.lighty.core/lighty-bom/badge.svg)](https://maven-badges.sml.io/maven-central/io.lighty.core/lighty-bom)
 [![License](https://img.shields.io/badge/License-EPL%201.0-blue.svg)](https://opensource.org/licenses/EPL-1.0)
 
 _This branch maintains compatibility with __OpenDaylight 2024-09 Scandium,__ release._