Execute request fails with input identifier as id attribute (id=identifier)

# Description
Using a an npm package for WPS on the client, the switch to WPS 2.0.0 (for advanced feature) produces
WPS Execute requests in the following form

**2.0.0 EXECUTE**
```
...

4

...
```

instead of this form (when using 1.0.0)

**1.0.0 EXECUTE**
```
...

pixel_size

56

...
```

I am not sure if this is a valid WPS request, but PyWPS execution fails when the first format is used with this error

```

area_of_interest

```

# Environment

- operating system:
- Python version: 3.8
- PyWPS version: 4.5.1
- source/distribution
- [ ] git clone
- [ ] Debian
- [x] PyPI
- [ ] zip/tar.gz
- [] other:
- web server
- [ ] Apache/mod_wsgi
- [ ] CGI
- [ ] other (gunicorn + nginx):

# Steps to Reproduce
Send a WPS Service EXECUTE request with the 2.0.0 format