fix: upgrade com.typesafe.akka:akka-cluster_2.13 from 2.6.21 to 2.8.7

Snyk has created this PR to upgrade com.typesafe.akka:akka-cluster_2.13 from 2.6.21 to 2.8.7.

See this package in maven:
com.typesafe.akka:akka-cluster_2.13

See this project in Snyk:
https://app.snyk.io/org/pantheon.tech/project/22a78fbe-a55f-412d-bb98-efcfd3bf82f0?utm_source=github&utm_medium=referral&page=upgrade-pr

lighty-core/dependency-versions/pom.xml

@@ -167,7 +167,7 @@
             <dependency>
                 <groupId>com.typesafe.akka</groupId>
                 <artifactId>akka-cluster_2.13</artifactId>
-                <version>2.6.21</version>
+                <version>2.8.7</version>
             </dependency>
             <dependency>
                 <groupId>com.typesafe.akka</groupId>