Update release template for flux2

See merge request openappstack/openappstack!417

Properly reference admin_email variable

See merge request openappstack/openappstack!424

Resolve "Rename prometheus-stack to kube-prometheus-stack"

Closes #823

See merge request openappstack/openappstack!422

Resolve "Cache docker images for CI"

Closes #422

See merge request openappstack/openappstack!412

Resolve "Use secrets from an .env file rather than from individual files"

Closes #804

See merge request openappstack/openappstack!416

Otherwise such errors like https://open.greenhost.net/openappstack/openappstack/-/jobs/89464
are not catched:

    $ sh ./install/install-openappstack.sh
    ✚ generating manifests
    ✔ manifests build completed
    ► installing components in flux-system namespace
    ✗ install failed: exec: "kubectl": executable file not found in $PATH
    ✚ generating GitRepository source
    ► applying GitRepository source
    ✔ GitRepository source created
    ◎ waiting for GitRepository source reconciliation
    ✔ GitRepository source reconciliation completed
    ✔ fetched revision: master/fe6c3302
    ✚ generating Kustomization
    ► applying Kustomization
    ✔ Kustomization created
    ◎ waiting for Kustomization reconciliation
    ✔ Kustomization openappstack is ready
    ✔ applied revision master/fe6c3302