fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU2004-NGHTTP2-5954795
- https://snyk.io/vuln/SNYK-UBUNTU2004-LIBSSH-6130560
- https://snyk.io/vuln/SNYK-UBUNTU2004-EXPAT-2403778
- https://snyk.io/vuln/SNYK-UBUNTU2004-EXPAT-2403934
- https://snyk.io/vuln/SNYK-UBUNTU2004-OPENSSL-3314788

Dockerfile

@@ -1,4 +1,4 @@
-FROM maven:3.6.3-jdk-11-openj9
+FROM maven:3.8.3-jdk-11-openj9
 
 ENV GEN_DIR /opt/openapi-generator
 WORKDIR ${GEN_DIR}