fix: stop exporting Spring Framework from the Spring module (#408) (CP: 5.0)

[vaadin-bot]

This PR cherry-picks changes from the original PR #408 to branch 5.0.

Original PR description

Summary

The plain Spring module no longer pulls in Spring Framework (spring-context) for the application. The application now declares Spring itself, so the application decides the Spring version, not the kit.

What changed

Behavior change: spring-context is now an optional dependency in observability-kit-spring, so it no longer comes in through the kit. This affects plain Spring apps (without Spring Boot) that use observability-kit-spring and got spring-context only through the kit. These apps must now declare spring-context themselves (plus spring-web for the HTTP observation hooks), best with org.springframework:spring-framework-bom imported.

• vaadin-spring stays a compile dependency of observability-kit-spring, so the module still brings it. Spring itself leaves vaadin-spring in feat(spring)!: stop exporting Spring dependencies at compile scope flow#25927.
• The starter is unchanged here. chore: move to version 25.4-SNAPSHOT for Vaadin 25.4 #413 already made spring-web a direct dependency of the starter, and fix: count resyncs from Flow events so the starter no longer needs spring-web #415 handles making it optional.
• The plain Spring test application now declares vaadin-spring directly.
• README: the plain Spring section now says the module brings vaadin-spring but not Spring, and tells users to declare Spring Framework and import its BOM.

🤖 Generated with Claude Code

[Artur-]

I don't think this should be in 5.0

[Artur-]

Or maybe this one can be - the overall change with Spring dependencies should not go into 25.3