FormLayout: consider making autoResponsive mode default

[rolfsmeds]

Describe your motivation

The new autoResponsive mode introduced to FormLayout in V24.8 is expected to be easier to use, less buggy, and more feature rich than the old responsiveSteps mode.

Yet, responsiveSteps remains the default for backward compatibility, which affects discoverability of new features and significantly diminishes the positive impact the new mode could have.

Describe the solution you'd like

Consider making autoResponsive mode the default in V25.0 or V26.0.

This would be a UI breaking change in the vast majority of Vaadin apps, as FormLayouts would now default to a single column with labels above fields regardless of layout width.

Getting back to the correct form layout could be done in a number of ways:

1. For each FormLayout, setAutoResponsive(false) in Flow;
2. For each FormLayout, in Flow setResponsiveSteps / in Hilla set responsiveSteps property to the old default;
3. A feature flag could revert all FormLayouts in the project to the old default.