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.

Start at StructlogFormatter.__call__ and inspect its event_dict type annotation. Reproduce the issue with the provided mypy example, update the annotation so it accepts the required mutable mapping type, and confirm that mypy no longer reports the list-item error.

Written by the indexing model from the issue text.