Don't show 'Free Trial of GitLab.com Gold' message for readonly DB's

What does this MR do?

When the GitLab Gold trial callout is dismissed on a Geo secondary, it results in a HTTP 403 and red error alert because a Geo secondary is not permitted to perform DB writes:

This MR updates EE::UserCalloutsHelper.show_gold_trial?() to also consider !::Gitlab::Geo.secondary_with_primary? as part of the checks.

I also swapped the order of checks used in _gold_trial_callout.html.haml from:

- if show_gold_trial? && user_default_dashboard?

to:

- if user_default_dashboard? && show_gold_trial?

because I benchmarked each method and found .user_default_dashboard?() to require less resources so it makes sense running it first.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8591

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #8591 (closed)