This annotation indicates that return value of the annotated method does
not need to be used. If it is _not_ present on a non-void method, and if
Error Prone's `CheckReturnValue` is active, then calling the method
without using the result is an error. However, we are not enabling
`CheckReturnValue` by default here.
Also update some code that does ignore return values, so that the
returned value is used, if only by assigning it to an unused variable.
* Port Junit assert to Truth in `com.google.gson.stream`
* Port Junit assert to Truth in `com.google.gson.regression`
* Port Junit assert to Truth in `om.google.gson.reflect`
* Port Junit assert to Truth in `com.google.gson.metrics`
* Port Junit assert to Truth in `com.google.gson.internal`
* Port Junit assert to Truth in `com.google.gson.internal.sql`
* Port Junit assert to Truth in `com.google.gson.internal.reflect`
* Port Junit assert to Truth in `com.google.gson.internal.bind`
* Port Junit assert to Truth in `com.google.gson.internal.bind.util`
* Port Junit assert to Truth in `com.google.gson.functional`
* Replaces `List.of` with `Arrays.asList` to grant legacy
* Simplify `==` asserts
* Simplify `.contain()` asserts + Minor fixes
* Simplify asserts
