Danger: Assign Backend reviewers for app/ & lib/ database files

What does this MR do?

Assign Backend reviewers too for app/ and lib/ Database related files

For example, in !61323 (comment 571085070) Danger only suggested database reviewers which I think is not accurate as that MR really affects key part of the Rails application

/cc @gitlab-org/maintainers/rails-backend @gitlab-org/maintainers/database