Deploy to Kubernetes cluster with KUBERNETES_DEPLOY_BRANCH condition

What does this MR do?

⚠️ Depends on Add job to rebase branches on main (!5497 - merged) • Georgi N. Georgiev | GitLab • 17.11 ⚠️

This change modifies the GitLab CI deployment configuration to make it more flexible. Previously, deployments to Kubernetes would only trigger when commits were made to the default branch (typically "main"). Now, deployments can also happen from a custom branch specified in the KUBERNETES_DEPLOY_BRANCH variable. This allows us to deploy from feature branches when needed, while maintaining the ability to deploy from "main" when no custom branch is specified.

gitGraph
    commit id: "HEAD"
    branch "feature/fault-tolerance"
    checkout main
    commit id: "New commit"
    checkout "feature/fault-tolerance"
    commit id: "Fault tolerance implementation"
    checkout main
    checkout "feature/fault-tolerance"
    merge main id: "Rebase new commit from main"
    commit id: "New commit rebased on fault tolerance"
    commit id: "Deploy to Kubernetes" type: HIGHLIGHT

Why was this MR needed?

We need this so we can deploy from feature/fault-tolerance.

What's the best way to test this MR?

What are the relevant issue numbers?

Related to [K8S Dogfooding] Use the fault tolerance featur... (#38458 - closed) • Georgi N. Georgiev | GitLab • 17.11