CIDR subnet should be changed when building Geo

What does this MR do?

When building 2 Geo instances with create_network option, user should specify unique subnet_cidr values. Otherwise there will be a clash like see in https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit-configs/staging-ref/-/issues/67#note_1341986854

Related issues

https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit-configs/staging-ref/-/issues/67#note_1341986854

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.