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 section titled "Spring Impersonation with Vaadin," the use of the class VaadinAwareSecurityContextHolderStrategy and the annotation @Import(VaadinAwareSecurityContextHolderStrategyConfiguration.class) needs clarification. Ensure that these elements are consistently referenced and correctly explained.

• There is a minor typo in the explanation under the note section for the "Spring Impersonation with Vaadin" where "If your are not using..." should be corrected to "If you are not using...".

• Review the placement of punctuation marks and ensure alignment with the standard American English conventions.

[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.