Adopting Spring Boot Multipart Configuration for File Uploads

### Feature description

Currently, file upload limits in Grails are configured as follows:

```
grails:
controllers:
upload:
maxFileSize: 20000000
maxRequestSize: 20000000
```

In Spring Boot, the equivalent configuration is:

```
spring:
servlet:
multipart:
max-file-size: 200MB
max-request-size: 200MB
```

I propose that we adopt the Spring Boot configuration properties instead.

This would provide several benefits:
- Avoid reinventing existing functionality
- Reduce redundant and framework-specific configuration code
- Improve readability by allowing the use of human-friendly size units (e.g., MB instead of raw byte values)

Start by tracing the existing `grails.controllers.upload` configuration entry point and how it applies `maxFileSize` and `maxRequestSize`; then compare it with Spring Boot's `spring.servlet.multipart` properties. Done means the Spring Boot settings control upload limits, support human-readable size units, and the redundant Grails-specific configuration is no longer required.

Written by the indexing model from the issue text.