[ux] Data source / target forms disallow AWS S3 access key and secret from environment

**Describe the bug**

In Label Studio 1.20, the data source and target UIs allowed blank entries for the AWS access key ID and secret to allow values to be read from the environment.

https://github.com/HumanSignal/label-studio/blob/665982e83fd57a311e10db1806bac408c2c82822/label_studio/io_storages/s3/form_layout.yml#L26-L42

With the new data manager UI, these fields are marked as being required, even though the tooltip says to leave them blank to read values from the environment.

**To Reproduce**
Steps to reproduce the behavior:

1. Configure AWS S3 with Access Key ID as a data source
2. Enter valid values in all fields except Access Key ID and Secret Access Key
3. Click Test Connection
4. The form reports the key ID and secret key fields are required, and will not use values from the environment.

**Expected behavior**
The form allows blank values and reads access key ID + secret from standard AWS sources (e.g., env vars, profile, ...)

**Screenshots**

**Environment (please complete the following information):**
- OS: Linux
- Label Studio Version 1.22.0

**Additional context**

Used to work as expected in 1.20.0. Haven't tried 1.21.0.