Update Grafana handling for 16.x

What does this MR do?

Follow up to !1090 (merged). That MR removed config lines referencing Grafana if targeting GitLab version 16.3 or higher. This isn't enough to pass the package pre-install check config, so an adjustment needed to be made.

This update sets monitor_enable_deprecated_grafana to true by default only if targeting a version < 16.0 (this matches Omnibus behavior, now). The behavior of monitor_install_bundled_grafana hasn't changed; it still inherits the value from monitor_enable_deprecated_grafana, although only if explicitly targeting a version < 16.3. For unspecified versions or 16.3+, it's set to false.

The grafana['enabled'] = false lines in gitlab.rb are now also included under that monitor_install_bundled_grafana flag. Anyone upgrading to 16.3 and using GET must first install 2.8.6, specify their current version AND set monitor_enable_deprecated_grafana to false, and then run all.yml to remove all of the configs. The upgrade can then proceed to 16.3.

Related issues

Closes #723 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: set up a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.