Small but important documentation change for secure S3 buckets

Hello,

I've successfully setup CNPG on a cloud where security is high by default.
To be able to connect to the storage as an S3 bucket I had to add
```
instanceSidecarConfiguration:
env:
- name: AWS_REQUEST_CHECKSUM_CALCULATION
value: when_required
- name: AWS_RESPONSE_CHECKSUM_VALIDATION
value: when_required
- name: AWS_DEFAULT_REGION
value: europe-3
```
to the `ObjectStore` config. The Barman Cloud docs are referring to the `checksum` vars but not the `region` var, which made it working.
Is it worth adding to the docs? According to the cloud provider tech support it is common for newer versions of AWS libs to require the region being set, so it could affect other S3 compatible storages as well.

Locate the Barman Cloud documentation covering S3-compatible storage and the ObjectStore instanceSidecarConfiguration example. Add AWS_DEFAULT_REGION alongside the documented checksum variables, then verify that the example and surrounding guidance explain when the region setting is needed.

Written by the indexing model from the issue text.