Ensure /mnt isn't used on Azure environments

What does this MR do?

MR switches out the use of /mnt on Azure specifically as it's a temporary directly on that platform. This isn't a standard approach but not one we can change it seems.

We used this directory in two places:

• NFS - Moved to /srv/gitlab-nfs. This should be a seemless switch on the next Ansible run as the files will be repopulated. If NFS is being used for Objects though instead of Object storage users will need to move this over beforehand.
• Gitaly Sharded - Moved to /var/opt/gitlab/git-data. If users are using Gitaly Sharded on Azure they will need to move their data. This is an annoyance but necessary as the Git data shouldn't be on a temp drive.

The directory use on all other cloud providers remains the same.

Related issues

Closes #629 (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.