reverting logback dependency update

[Mohit Jaggi]

Folks,
Due to a conflict with another tool we use, I can't use logback 1.2.3 and
slf4j 1.7.25 yet. Is it safe to change them to the previous values?

Ref:
https://github.com/apache/aurora/commit/d7425aa56d3fba98f4a16cb93bff8f9ce7ce0e67

Mohit.