[Java] version in the generated README dependent from {{java8}}

[developersteve]

Same issue as per swagger-api/swagger-codegen#7259 just need to update the README

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

[jmini]

To run OpenAPI-Generator itself you need Java 8. The main Reame in this project says so.

But the file you are modifying is for the Readme of the generated project. I did not check in detail, but I think that with some option you manage to get a generated project that can be built with "Java 6".

I think in the mustache readme the output should be dependent from if the java8 option is true or false.

[wing328]

@developersteve which Java API client (e.g. jersey2, okhttp-gson, etc) are you using? I believe most still work with Java 7.

[jmini]

@wing328 exactly I believe this too... Unless you set the java8 option during generation (See AbstractJavaCodegen.setJava8Mode(boolean) in the code).

Maybe we could/should make the README dependent of the {{java8}} variable.

[jmini]

I have update this PR to consider {{java8}}