Update dependabot reviewers to remove Rob

[fflaten]

PR Summary

Not really my place, but made an obvious update to remove Rob Holt as dependabot reviewer as he no longer works at Microsoft. 🙂

Should fix these types of errors: #1866 (comment)

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.