Adds default for prepared_statements config in Geo secondary db.yml

What does this MR do?

Rails is expecting prepared_statements to be either true or false, and prior to Rails 6.1, invalid values were "converted" to false, but in Rails 6.1, the interpreted value is either "false", or what is passed in the config literally.

This adds a default value set to false to the tracking DB, exactly as for the normal database, to prevent the invalid value.

Related issues

gitlab#333052 (closed) has more context around this

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
  • The outcome is that /opt/gitlab/embedded/service/gitlab-rails/config/database_geo.yml should contain prepared_statements: false by default
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened
  • We don't manually specify prepared_statements in the Chart so this doesn't seem necessary