Replace SKIP_MR option with DRY_RUN mode on redirects Rake task

What does this MR do and why?

While trying to resolve: gitlab!112520 (merged), I really needed a full "dry run" mode that made no local changes to my checkouts. I just needed to see what the script analyzed and be able to run bundle exec rake docs:clean_redirects repeatedly.

This MR:

• Replaces SKIP_MR mode (which just avoided pushes) with DRY_MODE (which won't made any local changes at all).
• Makes the docs:clean_redirects more robust by error handling nil in redirect_to frontmatter.
• Resolves Lint/NonAtomicFileOperation: https://docs.rubocop.org/rubocop/cops_lint.html#lintnonatomicfileoperation

CC @jglassman1 @ashrafkhamis @marcel.amirault

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Check out this branch.
3. Run DRY_RUN=true bundle exec rake docs:clean_redirects. If:

   • gitlab!112520 (merged) is already merged, the script will complete with no error but make no local changes.

   • gitlab!112520 (merged) isn't merged, the script will abort with this error message:

     INFO: (ee): In ../gitlab/doc/development/integrations/codesandbox.md, remove date: 2023-02-01 is less than today (2023-02-21).
     INFO: (ee): Not deleting ../gitlab/doc/development/integrations/codesandbox.md because running in dry run mode...
     
     ERROR: No redirect_to found in ../gitlab/doc/development/integrations/codesandbox.md!

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