Update Inc. contracts

close https://gitlab.com/gitlab-com/people-group/people-operations/General/-/issues/1163

US contracts have been updated. In an effort to standardize these contracts, there are now only four contract options:

1. Full-time: contains all compensation options.
2. Part-time, no stock
3. Federal LLC without clearance: contains all compensation options
4. Federal LLC w/clearance.

@gitlab-com/gl-ces Please review and let us know if this will work with your current process. We'd like to reduce the number of contract options, but want to make sure this proposal is feasible for your team. cc @pegan

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
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  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)