Move GitLab Database Loadbalancer from Premium to Free

Why is this change being made?

As described in https://gitlab.com/gitlab-com/Product/-/issues/2459 we want to open source the database loadbalancer to because

• The code would be available to a wider audience and could be moved into its own gem and the community will be able to use the LB (unlikely)
• Our engineers will have to write less specific code for LB because it is CE
• We will be able to use Rails 6.1 support for many databases and only rely on using our Load balancer for supporting many connections
• Some or all aspects of application-level sharding might be needed to be part of CE, since LB is a key low-level behavior of GitLab it makes it hard to support application-level sharding.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)