Update dependencies to address vulnerabilities.

JUnit 4.13.1 addresses a vulnerability in 4.13 -
https://github.com/junit-team/junit4/blob/HEAD/doc/ReleaseNotes4.13.1.md

Solr has a number of vulnerabilities which should be patched, the
main one being something that affects all versions under 8.4.0;

CVE-2019-17558 - Remote code execution in Solr 5.0.0 - 8.3.1
https://nvd.nist.gov/vuln/detail/CVE-2019-17558

To address this I've updated the dependency to be the latest version
(8.8.1).

pom.xml

@@ -34,7 +34,7 @@
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
         <!-- the Solr version used in dependency section for all related dependencies -->
-        <solr.version>6.6.6</solr.version> 
+        <solr.version>8.8.1</solr.version>
         <!-- the Jetty version used in dependency section for all related dependencies -->
         <jetty.version>9.4.35.v20201120</jetty.version>
                 
@@ -366,7 +366,7 @@
 		    <dependency>
 		        <groupId>junit</groupId>
 		        <artifactId>junit</artifactId>
-		        <version>4.13</version>
+		        <version>4.13.1</version>
 		        <scope>test</scope>
 		    </dependency>