Update version of third party dependency `scrooge_generator` to version 20.9.0

[cattibrie]

Problem

rules_scala uses 2 years old version (18.6.0) of scrooge_generator.
Keeping third party dependencies up-to-date is healthy for the project.

Main motivation: Twitter uses the latest version of scrooge.
Currently, rules_scala do not support a lot of features that are available in the latest scrooge for Twitter developers, e.g. parsing of the compiler_args, --java_passthrough flag doesn't exist in the version 18.6.0 of scrooge-generator.

Solution

Update scrooge to the latest version and keep it up-to-date.
Twitter is happy to be responsible for keeping this third party up-to-date.

[wisechengyi]

For context there shouldn't be any breaking changes since 18.6.0 per https://twitter.github.io/scrooge/changelog.html

[ittaiz]

Sounds good to me. We should probably wait a bit with the merge to give people time to react (and well done on the slack message) but I think we should do it.