The "stable version should not decrease" formula audit currently
prevents us from being able to create bottles when downgrading a
formula version. We previously worked around this by bumping
`version_scheme` but this wasn't an intended use case and we now
avoid using it for this purpose.
We can handle simple formula downgrades by reverting changes in a
syntax-only PR but that isn't sufficient when we need new bottles
(i.e., if additional changes have been made to the formula in the
interim time). In the latter case, the only available solution may be
to revert all changes made after the previous version using a
syntax-only PR and then create another PR to reintroduce the other
changes and create new bottles.
To avoid the aforementioned approach, this splits the stable version
audit into a separate method, so we can use `brew audit
--except=stable_version` to selectively skip it.
ccbde5952d