[web] rename panel color family to frame

Summary:
Rename diff. In D10500 I introduced a new "panel" component. Now that we have this new component called "panel" this panel color family does not make sense. In a comment on D10500 I explained that we should rename the panel color family to "frame" and then reintroduce the panel color family in the context of this new panel component (those color variables are currently under the "card" color family and will be updated in a subsequent diff)

Link to comment: https://phab.comm.dev/D10500?id=35172#303769

Test Plan: flow + confirmed no visual regressions

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10514