StructlogProcessor fails typechecking

When typechecking the following code:

```python
from structlog.typing import Processor

shared_processors: tuple[Processor, ...] = (
structlog.contextvars.merge_contextvars,
structlog.processors.add_log_level,
structlog.processors.StackInfoRenderer(),
structlog.dev.set_exc_info,
structlog.processors.TimeStamper(fmt="iso", utc=True),
)

processors: list[Processor]
if sys.stderr.isatty():
processors = [
*shared_processors,
structlog.dev.ConsoleRenderer(),
]
else:
processors = [
*shared_processors,
structlog.processors.dict_tracebacks,
ecs_logging.StructlogFormatter(),
]

structlog.configure(
processors=processors,
logger_factory=structlog.PrintLoggerFactory(),
cache_logger_on_first_use=True,
)
```

mypy raises the following:

```
error: List item 2 has incompatible type "StructlogFormatter"; expected
"Callable[[Any, str, MutableMapping[str, Any]], Union[Mapping[str, Any], str, bytes, bytearray, tuple[Any, ...]]]" [list-item]
ecs_logging.StructlogFormatter(),
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: "StructlogFormatter.__call__" has type "Callable[[Arg(Any, '_'), Arg(str, 'name'), Arg(dict[str, Any], 'event_dict')], str]"
```

`StructlogFormatter` is annotated as accepting a `dict[str, Any]`, but it needs to accept any `MutableMapping[str, Any]`. The code already conforms to this, so addressing this issue wil hopefully only involve updating the type annotation.