docs: add upload format change to upgrade guide

[sissbruecker]

Mention the upload format changes from vaadin/web-components#10424, vaadin/web-components#10423 in the upgrade guide.

[github-actions]

AI Language Review

• In the "Upgrading Guide" file, the following section title could be improved for consistency in capitalization: "Upgrade To VaadinSecurityConfigurer From VaadinWebSecurity" should be "Upgrade to VaadinSecurityConfigurer from VaadinWebSecurity".

• In "Theme Structure" under "Frontend-based Add-ons", consider rephrasing "depending on which theme was applied using the @theme annotation" for better readability.

• The note on why Lumo styles are not included by default anymore in "Lumo Theme" could include an explanation on how users can achieve similar effects, focusing on newer best practices.

• In "WebComponentExporter", some sentences can be made clearer by not nesting too many details in one sentence.

• In several sections, such as "Maven & Gradle Plugins", replace quotes around versions (e.g., "Java 21") with consistent formatting for clarity.

• In the "Spring Boot 4" and "Java 21" sections, ensure clarity on any potential required changes to users' current configurations to facilitate easier transitions.

• It would be beneficial to include an overview or summary at the beginning of "Upgrading Add-ons" to set expectations about what each detailed subsection will cover.