docs: Sphinx `>=7.4.0` breaks warning ignore script

# Problem
The logging in sphinx changed slightly in `7.3.7` -> `7.4.0`. This breaks our `warnings_filter.py`-script, that helps to ignore false positives. The issue persists until at least `8.1.3`.

# Details
It is clear, why this set of warnings does not work and others (e.g. `Duplicate C++ declaration`, ...) do. There was a change in Sphinx regarding the handling of the messages from:
```
logger.warning(msg % target, ...
```
to
```
logger.warning(msg, target, ...
```
. This might change the time, when the expression is evaluated.

### Message in log
```
/home/runner/work/acts/acts/docs/api/api.md:60: WARNING: undefined label: 'class_acts_1_1_convex_polygon_bounds_3_01_polygon_dynamic_01_4'
/home/runner/work/acts/acts/docs/api/api.md:171: WARNING: undefined label: 'class_acts_1_1_axis_3_01_axis_type_1_1_equidistant_00_01bdt_01_4'
/home/runner/work/acts/acts/docs/api/api.md:171: WARNING: undefined label: 'class_acts_1_1_axis_3_01_axis_type_1_1_variable_00_01bdt_01_4'
/home/runner/work/acts/acts/docs/api/api.md:177: WARNING: undefined label: 'class_acts_1_1_grid_surface_material_t'
```

### Handled object until `7.3.7`
```




```

### Handled object since `7.4.0`
```

```

# Possible solutions
## Stick to Sphinx `7.3.7`
### Pro
- No extra work needed
- We directly see, when we generate new warnings of the type `undefined label`

### Con
- Already old (Apr 19 2024), which might lead to problems in the future
- Locked in to current dependencies, which might need updates in the future

## Upgrade to Sphinx `8.1.3` and ignore all `undefined label:`-warnings
Just add `.*undefined label: .*` to the regex-filter.

### Pro
- Newest version including bugfixes
- no constraints in upgrading Sphinx or any of it dependencies

### Con
- We won't see new warnings including `undefined label:` immediately