[BUG] Build failure with OpenJDK 16

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

I would like to contribute to openapi-generator's language support, but I'm not very familiar with Java, and the build is failing on my system.

❯ mvn clean install -e

[INFO] Running TestSuite
Configuring TestNG with: TestNG60Configurator
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
java.lang.instrument.IllegalClassFormatException: Error while instrumenting sun/util/resources/provider/NonBaseLocaleDataMetaInfo.
	at org.jacoco.agent.rt.internal_43f5073.CoverageTransformer.transform(CoverageTransformer.java:94)
...

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.1.0:jar (attach-javadocs) on project openapi-generator-core: MavenReportException: Error while generating Javadoc: Project contains Javadoc Warnings -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.1.0:jar (attach-javadocs) on project openapi-generator-core: MavenReportException: Error while generating Javadoc: Project contains Javadoc Warnings
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:215)

openapi-generator version

master (2e85ccdec8c3304e42c3434ef6da54a637a13e1a)

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

• clone https://github.com/OpenAPITools/openapi-generator.git
• mvn clean install -e

❯ java --version
openjdk 16.0.1 2021-04-20
OpenJDK Runtime Environment AdoptOpenJDK-16.0.1+9 (build 16.0.1+9)
OpenJDK 64-Bit Server VM AdoptOpenJDK-16.0.1+9 (build 16.0.1+9, mixed mode, sharing)

❯ mvn --version
Apache Maven 3.8.1 (05c21c65bdfed0f71a2f2ada8b84da59348c4c5d)
Maven home: /usr/local/Cellar/maven/3.8.1/libexec
Java version: 16.0.1, vendor: Homebrew, runtime: /usr/local/Cellar/openjdk/16.0.1/libexec/openjdk.jdk/Contents/Home
Default locale: en_CA, platform encoding: UTF-8
OS name: "mac os x", version: "11.4", arch: "x86_64", family: "mac"

Related issues/PRs

#9804 a different build error

Suggest a fix

Start by reproducing the failure with mvn clean install -e under OpenJDK 16 and inspect the JaCoCo instrumentation and maven-javadoc-plugin errors reported for openapi-generator-core. The issue is done when the project builds successfully with the stated Java and Maven versions.