Use dependabot to keep dependencies up to date

[rjmholt]

PR Summary

Move to using dependabot to manage PSSA's dependencies.

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.

[bergmeister]

Can do but to prevent noise, we might want to exclude certain dependencies like Newtonsoft, PowerShell Standard and other .net dependencies that cannot be updated to the latest version due to the tight coupling with the oldest LTS version of PowerShell Core. Unfortunately with that, we'd exclude the majority of dependencies.

[JamesWTruher]

I think this is a good idea - most of the other PowerShell repositories are using it

[JamesWTruher]

very nice