add to ignored mockito methods in `AbstractReturnValueIgnored`: `org.mockito.BDDMockito#then`

> ATTENTION! Please read and follow:
> - if this is a _question_ about Error Prone, send it to error-prone-discuss@googlegroups.com
> - if this is a _bug_ or _feature request_, fill the form below as best as you can.

### Description of the problem / feature request:

Feature request: add to ignored mockito methods in `AbstractReturnValueIgnored`: `org.mockito.BDDMockito#then`

### Feature requests: what underlying problem are you trying to solve with this feature?

False detection of ReturnValueIgonored in tests using BDDMockito

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

```
Supplier mockSupplier = Mockito.mock(Supplier.class);
BDDMockito.then(mockSupplier).should().get();
```

### What version of Error Prone are you using?

2.3.4

### Have you found anything relevant by searching the web?

Existing code in errorprone's AbstractReturnValueIgnored already caters for similar method, e.g. verify()