Document DaoAuthenticationProvider timing attack mitigation

[AzazelSensei]

Closes gh-19082

Documents that DaoAuthenticationProvider still runs password validation through the configured PasswordEncoder when a username is not found (dummy encoded password + matches), so the not-found path stays roughly in the same time range as a found path using that encoder.

Also documents the limitation when a credential store mixes hash algorithms: Spring Security cannot pick one not-found duration that matches every found path, and upgrading stored passwords to a modern algorithm is the practical mitigation.

Changes

• Short note on the DaoAuthenticationProvider reference page
• New Servlet exploits page on timing attacks, linked from the exploits index and nav

Verification

• Cross-checked against DaoAuthenticationProvider#retrieveUser, prepareTimingAttackProtection, and mitigateAgainstTimingAttack
• Docs describe only the behavior present in that implementation

[AzazelSensei]

Superseded by clean branch: #19533