fix: Replace deprecated ACL.SYSTEM with ACL.SYSTEM2 in doCheckCredentialsId() and migrateCredentials()

[ArpanC6]

Summary

Replace deprecated ACL.SYSTEM with ACL.SYSTEM2 in two methods in MailAccount.java.

Changes

• Line 173: Replaced ACL.SYSTEM with ACL.SYSTEM2 in doCheckCredentialsId()
• Line 313: Replaced ACL.SYSTEM with ACL.SYSTEM2 in migrateCredentials()

Testing

No functional change, only deprecation fix.
This follows up on the same fix applied in #1494 for doFillCredentialsIdItems().

[ArpanC6]

The CI failures are pre-existing issues unrelated to this change:

1. Jenkins Security Scan: Build fails in MailAccount.java due to org.acegisecurity.Authentication → org.springframework.security.core.Authentication incompatibility (pre-existing, not introduced here)
2. JUnit step failure: Pre-existing issue on linux-25, also present on other PRs

This PR only replaces ACL.SYSTEM with ACL.SYSTEM2 in two methods — no functional change.