allow user to set kube-rbac-proxy image

[pdabelf5]

Proposed changes

• allow user to set kube-rbac-proxy image & tag via Makefile variables KRP_IMAGE_BASE & KRP_IMAGE_TAG
• set containerImage value when VERSION variable changes
• make it clear which values in the templates are placeholders
• bump minimum kubernetes version to 1.26

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork