fix(handlers): disable upload purging if read-only mode is enabled (#169)

mentioned in commit 6d2ec3d1

mentioned in merge request gitlab-org/build/CNG!827 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1363 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1367 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2285 (merged)

mentioned in merge request omnibus-gitlab!5827 (merged)

mentioned in merge request gitlab-development-kit!2385 (merged)