[java] Specify Java version for maven-javadoc-plugin (fixes Java 11 error)

[aloisklink]

Specify the Java version to maven-javadoc-plugin. This is essentially just adding the <source>java.version</source> tag to pom.mustache files, e.g.:

<artifactId>maven-javadoc-plugin</artifactId>
<version>3.1.1</version>
<configuration>
  <doclint>none</doclint>
  {{#supportJava6}}
    <source>1.6</source>
  {{/supportJava6}}
  {{^supportJava6}}
  {{#java8}}
    <source>1.8</source>
  {{/java8}}
  {{^java8}}
    <source>1.7</source>
  {{/java8}}
  {{/supportJava6}}

for almost all clients, except java/rest-assured, which I hard-coded to <source>1.8</source>, since it only supports Java 8.

This fixes the same issue as #5361 when trying to use generated clients in Java 11:

An error has occurred in Javadoc report generation: [ERROR] Exit code: 1 - javadoc: error - The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module error that happens when using Java 11

Additionally, I've fixed the broken integration tests in java/retrofit2-play24 and java/jersey2-java6. java/feign tests are also failing, but that should be fixed by the already pending pull request #5786.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
  • No breaking changes (as far as I'm aware)
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.
  • @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @bkabrda (2020/01)

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[wing328]

@aloisklink thanks for the PR. Can you please resolve the merge conflicts when you've time?

[aloisklink]

Hi @wing328, cheers letting me know about the merge conflicts! I swear Github used to send out alerts if there was a merge conflict, but maybe I was imagining it.

[wing328]

@aloisklink thanks for the PR.

Merged via #6679 (with merge conflicts resolved)