fix: pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-COMGOOGLEGUAVA-5710356
- https://snyk.io/vuln/SNYK-JAVA-IONETTY-5725787

pom.xml

@@ -53,7 +53,7 @@
     <mockito.version>4.11.0</mockito.version>
     <wiremock.version>3.0.0-beta-2</wiremock.version>
     <pulsar.group.id>io.streamnative</pulsar.group.id>
-    <pulsar.version>3.0.0.1</pulsar.version>
+    <pulsar.version>v2.8.0-rc-202106091140</pulsar.version>
     <slf4j.version>1.7.25</slf4j.version>
     <spotbugs-annotations.version>3.1.12</spotbugs-annotations.version>
     <apicurio.version>2.1.3.Final</apicurio.version>