Default file upload limit per form message is unconditionally required in global configuration

Problem/Motivation

Following the changes in #3543014: Make form_file_limit error message configurable, a new global configuration field was added for Default file upload limit per form message.

However, this field is currently set with '#required' => TRUE unconditionally. As a result, administrators attempting to save global element settings (/admin/structure/webform/config/elements) are blocked from saving any changes unless a message is provided—even when no global file upload limit (default_form_file_limit) is configured.

Steps to reproduce

1. Navigate to Structure > Webforms > Configuration > Elements (/admin/structure/webform/config/elements).
2. Ensure Default file upload limit per form is empty.
3. Leave Default file upload limit per form message empty.
4. Attempt to save the form (or alter any unrelated setting, such as CSS classes).
5. Result: Form validation fails with a required field error on Default file upload limit per form message.

Proposed resolution

Update the form definition for default_form_file_limit_message in the configuration form:
1. Remove '#required' => TRUE.
2. Add #states to make the field conditionally required and visible only when default_form_file_limit is filled.

Remaining tasks

User interface changes

API changes

Data model changes