Deprecate the Helm Chart docker-registry-secrets

[dduportal]

This charts has been used to provide Docker Registry credentials for our Kubernetes pods so they can pull images from the Docker Hub without hitting the rate limit of unauthenticated Docker accounts.

• Agents are only using jenkinsciinfra/* and jenkinsci/* images from the DockerHub. Since the past 18 months, we've been sponsored by DockerHub, these images are not subject to any rate limit: DockerHub rate limiting #2961

• Datadog (explicit setup of the chart) it uses GCR registry instead of Datadog by default since at least 2 years. We already validated this on publick8s cluster since 14 months with success. We can safely stop providing the Docker registry secret.

Task list:

• Remove helmfile release from kubernetes management
• Uninstall existing charts and secrets
• Deprecate the chart from jenkins-infra/helm-charts code repository

[dduportal]

After merging jenkins-infra/kubernetes-management#6034, ran the following command on each cluster to cleanup all dangling resources:

helm uninstall -n default docker-registry-secrets

[dduportal]

Cleaned up the SOPS secret in: https://github.com/jenkins-infra/charts-secrets/commit/38947fe5b5f844e90b89026ad757180126e647ac

Note: we use the Docker registry secret on other locations, so no use to delete it in DockerHub