fix: order of API paths in proto for AssignMultipleGroups

[mapmeld]

Description

Calls to PATCH /api/v1/users/assignments, added in #7971 , are failing because it was defined in the proto after /api/v1/users/{user_id}

Test Plan

Explains error on latest-main: type mismatch, parameter: user_id, error: strconv.ParseInt: parsing \"assignments\": invalid syntax

URL is not conflicted by any other definitions in api.proto

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

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