Update Java Style Guide for Java 17

The next Java LTS, Java 17 is coming later this year.

There is no information on which version of Java the **Google Java Style Guide** adheres to.
It doesn't look like it has been updated to incorporate the new language features that has been added to newer Java.

There is nothing about Java modules mentioned in the style guide.
There is nothing about the new language features, like `var`, `Switch Expressions`, `Records`, `Text Blocks`, `Sealed Classes`, `Hidden Classes`, `Pattern matching instanceof`.
Though some of these new features are still in First or Second Preview as of Java 16 coming in March 2021.