Upgrade CircleCI JDK8 to a newer version to catch JavaDoc errors

Description

Current JDK8 in CircleCI build won't detect the following Javadoc errors:

[ERROR] /Users/williamcheng/Code/openapi-generator/samples/client/petstore/java/rest-assured/src/main/java/org/openapitools/client/api/UserApi.java:271: error: unknown tag: User
[ERROR]          * @param user (List<User>) List of user object (required)
[ERROR]                             ^
[ERROR]
[ERROR] Command line was: /Library/Java/JavaVirtualMachines/jdk1.8.0_172.jdk/Contents/Home/jre/../bin/javadoc @options @packages
[ERROR]
[ERROR] Refer to the generated Javadoc files in '/Users/williamcheng/Code/openapi-generator/samples/client/petstore/java/rest-assured/target/apidocs' dir.
[ERROR]
[ERROR] -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

Current JDK version in CircleCI:

openjdk version "1.8.0_141"

openapi-generator version

Latest master

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/697#issuecomment-409573524

Suggest a fix/enhancement

Upgrade JDK to a new version (8.x)