Switch Pod calculations to be based on env size

What does this MR do?

MR changes how GET calculates Pod counts to set in Cloud Native Hybrid environments.

Previously we calculated based on available resources in the node pools. However with !458 (merged) node pools will now autoscale and can no longer be used as the pin for these calculations.

This MR switches to deploying pods by environment size, specifically Gitaly size. GET will now deploy Webservice and Sidekiq nodes that are enough to match the Gitaly size based on calculations from the Reference Architectures. Worth noting this is just the default values, users can still override pod counts directly.

Related issues

Relates #353 (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
• 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: setup 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.