[red-knot] Remove `Type::Unbound` (#13980)
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## Summary
- Remove `Type::Unbound`
- Handle (potential) unboundness as a concept orthogonal to the type
system (see new `Symbol` type)
- Improve existing and add new diagnostics related to (potential)
unboundness
closes #13671
## Test Plan
- Update existing markdown-based tests
- Add new tests for added/modified functionality
