[Java.Base] Flush Java.Base-ref.cs

Context: #1288

We're considering adding a check to CI if the build changes any files,
and `src/Java.Base-ref.cs` is one of the primary instigators for such
in-tree changes.

Flush the current output of `Java.Base-ref.cs` when building against
JDK-11, which is the current JDK used on dotnet/java-interop CI.

Author: jonpryor