Update default sandbox name in cleanup scripts to include day of week

What does this MR do and why?

Part of https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1315

This MR updates the default sandbox_name cleanup scripts to use top-level sandbox group based on day of week.

Update:

Added a commit to increment wday by 1 since Sunday translates to 0

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.