test: prepare for Lagoon v2.20.0 release

[tobybellwood]

This PR is a pre-release branch for Lagoon v.2.20.0

It also incorporates a number of other improvements:

• automated next-release label testing to remove the need to. manually set testlagoon&main for images
• updated Kubernetes version to 1.30 and associated packages
• Sets the minimum supported version of Kubernetes to 1.25 (as per some of the upstream charts for other components)
• Streamlines the install process for the lint/test routines to use built-in binaries where possible.
• It incorporates the local-development changes from https://github.com/uselagoon/lagoon/pull/3710