refactor: use generic function sliceContains

[alexandear]

This PR refactors the code to utilize the generic function sliceContains from the pkg/codegen package.

Upon migration to Go 1.21, sliceContains can be substituted with slices.Contains.

[jamietanna]

Hi, appreciate you've worked on this and it's not yet in - please bear with, we're over-stretched and are trying to prioritise higher priority pieces (when we get time to work on the project).

Once we get through the backlog for the next release we'll revisit this

[alexandear]

Closing due to inactivity.