Remove MySQL Support

Background

We already state that MySQL support is deprecated.

We've already agreed that we will remove MySQL support at some point. This issue outlines the current plan to do so.

Costs of Ongoing MySQL Support

There is ample discussion about the costs of ongoing support in https://gitlab.com/gitlab-org/gitlab-ce/issues/51173

Proposal

Remove support in 12.1

Rationale

Less than 1% of all instances use MySQL. Only about 0.25% of Starter/Premium/Ultimate customers use MySQL.

Tasks

• Summarize Costs of continued MySQL Support in https://gitlab.com/gitlab-org/gitlab-ce/issues/51173
• Cleanup existing language around deprecation - @kencjohnston
• Communicate Plan with GitLab Customer Success, Support Teams - Confidential issue, customer names
• Communicate Plan with Community