Use self-documenting f-strings where possible

Could maybe be added to `flake8-simplify`.

```py
my_var = 42_000

# bad (if target=py3.8+)
f"my_var={my_var}"
f"my_var = {my_var}"
f"my_var = {my_var:,}"

# good
f"{my_var=}"
f"{my_var = }"
f"{my_var = :,}"
```

Review the flake8-simplify proposal and Ruff's existing lint-rule entry points for Python f-strings. Confirm how the target Python version is represented and how similar suggestions are tested. Done means eligible f-strings receive a diagnostic recommending self-documenting syntax, including the formatting examples shown.

Written by the indexing model from the issue text.