Fix: Address User-Controlled Data Risk in isAuthenticated()

[gantoin]

Related to https://github.com/jhipster/generator-jhipster/issues/27051

For enhancing application security, this PR modifies the isAuthenticated() endpoint to return a boolean indicating the authentication status instead of a potentially user-controlled value (principal.getName()). This change mitigates the risk of reflecting user-controlled data and strengthens the security of the application.

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[mraible]

Any updates? Builds are currently failing.

[gantoin]

I guess https://github.com/jhipster/generator-jhipster/issues/27051 is fixed by https://github.com/jhipster/generator-jhipster/pull/28729 @mraible