Stop using DV to mean simulation

Nobody has claimed this yet.

Part of lowRISC/dvplan#129.

DVSim uses DV to mean simulation in places, which stops working as soon as a formal flow is part of the same picture. DV covers both, and the bundle config added under lowRISC/dvplan#129 takes `flow: dv` for exactly that reason.

## Scope

- Sweep the prose, docstrings, help text and log messages for DV used to mean simulation, and say simulation where simulation is meant. `flow: sim` keeps its name, since it does name a simulation.
- `doc/dv_evidence.md` needs care. The format name `lowrisc-dv-evidence` is written into files that already exist, so the name stays. What changes is the prose describing it as what a simulation flow writes, since a formal run can produce one too.
- Directory names such as `hw/ip//dv/` belong to OpenTitan and are out of scope.

## Done when

- No user-facing string in DVSim uses DV to mean simulation.
- `doc/dv_evidence.md` describes a producer as a regression flow of any kind.

Search DVSim's prose, docstrings, help text, and log messages for uses of DV that mean simulation, while leaving flow: sim unchanged. Read doc/dv_evidence.md carefully, preserving the lowrisc-dv-evidence format name but updating its description. Done means no user-facing string uses DV to mean simulation and the document describes producers as regression flows of any kind.

Written by the indexing model from the issue text.