Use Java11 by default

[danielpetisme]

Is your feature request related to a problem? Please describe.
Java8 is EOL end of 2020, plus you can't pretend to be a hipster if you're still on Java8

Describe the solution you'd like
Use Java11 and the related language feature rather than Java 8

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Waiting for this PR
quarkusio/quarkus#5691

[danielpetisme]

Currently the generator use Java11 without any identified issue at least on JVM mode.

Further tests should be done on native mode to know if we can close this issue

[danielpetisme]

Well looks like Quarkus support java11 by default now (even if the upstream issue is open...)
https://quarkus.io/blog/quarkus-1-3-1-final-released/

[danielpetisme]

Using Java11 since a month no issue reported...