Add a US Federal exception to requirements of OSS program

Why is this change being made?

There isn't a consensus on how to deal with US Federally affiliated open source projects yet. In some cases, Sales team members have wanted us to grant OSS licenses, and in some cases, Sales team members have asked us not to issue an OSS license. See relevant issue: https://gitlab.com/gitlab-com/marketing/community-relations/opensource-program/general/-/issues/89

In order to make this more fluid, this merge request changes the language of the OSS program requirements section to make it clear to applicants that not all US Fed open source projects will qualify. This gives us more transparency around our US Fed exception practice, and also gives us more control.

Under this new process, the OSS team would flag all US-federally affiliated orgs as leads for Sales and would work with them to decide whether or not to grant an OSS license. Those accepted would follow the formal OSS exception process: https://gitlab.com/gitlab-com/marketing/community-relations/opensource-program/general/-/issues/121

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