docs: mention removal of classNameGenerator API in Grid

[vursen]

Follow-up to

• refactor!: remove deprecated classNameGenerator API flow-components#8259
• refactor!: remove deprecated classNameGenerator API web-components#10502

[github-actions]

AI Language Review

The file "Upgrading Guide" has been updated with the following improvements:

1. Additional Context in Grid Component Section: The section on Grid components has been expanded to include details about the removal of deprecated methods setClassNameGenerator and getClassNameGenerator. These methods are replaced with the setPartNameGenerator method and the cellPartNameGenerator property, respectively. These additions provide clarity on how to update code in response to these changes.

2. Formatting and Consistency: The document maintains consistent formatting and style throughout. However, ensure that all headings and sections use title case for consistency and professionalism.

3. Detailed Instructions: The guide provides detailed instructions regarding the use of new APIs, which are necessary for developers to successfully upgrade their Vaadin applications. Ensure there are no typos or misunderstandings in the new sections, particularly with coding examples.

Overall, the modifications better guide developers on making necessary changes when upgrading their applications to Vaadin 25. Ensure continued adherence to the rest of the provided guidelines for clarity and ease of understanding.