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

Change-Id: Ic3f7f448e020733d8545ceb6c3204de222b870d1
Signed-off-by: tobias.pobocik <[email protected]>

Author: Tobianas

README.md

@@ -1,4 +1,4 @@
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/org.opendaylight.odlparent/odlparent/badge.svg)](https://maven-badges.herokuapp.com/maven-central/org.opendaylight.odlparent/odlparent)
+[![Maven Central](https://maven-badges.sml.io/maven-central/org.opendaylight.odlparent/odlparent/badge.svg)](https://maven-badges.sml.io/maven-central/org.opendaylight.odlparent/odlparent)
 [![javadoc](https://javadoc.io/badge2/org.opendaylight.odlparent/odlparent/javadoc.svg)](https://javadoc.io/doc/org.opendaylight.odlparent/odlparent)
 [![License](https://img.shields.io/badge/License-EPL%201.0-blue.svg)](https://opensource.org/licenses/EPL-1.0)