Should the java compiler version be unified as 1.8 or newer

[letuer]

There is two java compiler version in master branch，1.7 and 1.8.

Should we upgarde it to 1.8 or newer. For the following reasons:

1. Java8 and 11 is most popular Java version。Few applications still use Java7。 See 2020-java-technology-report.
2. Java8 new features, like lambda、stream，can improve development efficiency， make code implementation cleaner。

Please think about it. Thanks.

[ejona86]

Duplicate of #4671.

Note that there is nothing to "unify" here. The 1.8s you see there are all Android, which can do language desugaring for older platforms but isn't actually Java 8 (e.g., stream is not available; although there is also some library desugaring available). But why it is needed is because of some fallout from Guava warning about dropping Java 7 support (see https://github.com/grpc/grpc-java/releases/tag/v1.38.0 and https://github.com/google/guava/releases/tag/v30.1).

[letuer]

Thanks, I get it. If not necessary, compatibility for older platforms is more important.