fix: Fix previous_version method to return previous minor patch

This makes a change to previous_version to return the expected version.

We expect this method to return 'the latest patch of the previous minor version.

i.e if:

• the current version is 16.4.x
• the previous minor was 16.3
• then the #previous_version should return 16.3.x

Closes gitlab-com/gl-infra/delivery#19692 (closed)