build(deps): update module github.com/spf13/viper to v1.20.0

mentioned in commit 99ff18b5

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!4245 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!4244 (merged)

mentioned in merge request gitlab-org/charts/gitlab!4184 (merged)

mentioned in merge request gitlab-org/build/CNG!2290 (merged)

mentioned in merge request omnibus-gitlab!8268 (merged)