Change instance type and os_boot_disk size for Praefect servers

While we are upgrading the OS on praefect servers, it also presents us with an opportunity to change the instance type for cost savings, give that praefect servers in production are over-provisioned. At the same time, it also gives us a window to update OS boot disk size on the praefect servers from 20GB to 50GB in gstg and gprd.

Checklist

• Update OS boot disk size in gstg from 20GB to 50GB to avoid potential disk saturation 👉 https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/18057#note_1318556767
• Update instance type of praefect servers in gstg to t2d-standard-2 to match instance types with production 👉 https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/18057#note_1318556767
• Update OS boot disk size in gprd from 20GB to 50GB to avoid potential disk saturation 👉 https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/18057#note_1332375682
• Update instance type of praefect servers in gprd to t2d-standard-8 to bring in costs savings 👉 https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/18057#note_1332375682