3.0: Remove Session#doPrivileged; switch to Subject.doAsPrivileged

[arturobernalg]

• Remove the Security Manager–dependent Session#doPrivileged from jspwiki-api.
• Replace the only usage with Subject.doAsPrivileged in DefaultAuthorizationManager; behavior unchanged.
• Keeps policy checks: global via AccessController, local via LocalPolicy.
• Rationale: avoid a 3.x→4.x API break; aligns with JDK 17–25 where SM is deprecated/disabled.
• Migration: callers should use Subject.doAs( session.getSubject(), action ) (or doAsPrivileged if needed).