docs: refactor impersonation example

[mcollovati]

Refactors impersonation example to get the correct security context holder as a parameter, to avoid wiring the dependency by name.

[github-actions]

AI Language Review

• In the "Spring Impersonation with Vaadin" section, the updated instruction for creating the SwitchUserFilter bean introduces complexity by injecting the VaadinAwareSecurityContextHolderStrategy directly. Clarify if this adjustment affects existing configurations, and consider whether this inclusion is necessary in the context of projects not using VaadinWebSecurity as a base class.
• The note about adding @Import(VaadinAwareSecurityContextHolderStrategyConfiguration.class) lacks grounding. Provide more context or scenarios where this import is critical, assisting readers in understanding why this step could be necessary.

[heruan]

I would suggest to inject the strategy as the interface SecurityContextHolderStrategy, in case the user provides their own strategy bean.

[mcollovati]

The filter needs VaadinSecurityContextHolderStrategy to work properly with Vaadin. A different strategy (e.g. ThreadLocal) will potentially fail.

[mcollovati]

Added a note suggesting to add @Import(VaadinAwareSecurityContextHolderStrategyConfiguration.class) if application is not using VaadinWebSecurity as a base configuration class.