Remove bundled Grafana chart and related configuration

What does this MR do?

Bundled Grafana chart was deprecated in 15.10 - gitlab-org/gitlab!114955 (merged). Let's remove it, and let users know about it by a deprecation message.

Test plan

1. Default deployments (i.e those without global.grafana at all - those using basic values file) gets upgraded without any warnings.

2. Deployments with global.grafana.enabled set to true will fail to upgrade and show the following warning

   DEPRECATIONS:
   
   grafana:
       The bundled Grafana chart has been removed, and thus `global.grafana.enabled` does not have any effect. It is recommended that you switch to the newer chart version from Grafana Labs available at https://artifacthub.io/packages/helm/grafana/grafana or a Grafana Operator from a trusted provider. You can find instructions to integrate Grafana with GitLab at https://docs.gitlab.com/ee/administration/monitoring/performance/grafana_configuration.html.

3. Deployments with global.grafana.enabled set to false gets upgraded without any warnings.

Related issues

Closes #4353 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.