chore(clients): suppress unavoidable X509Certificate DN deprecation warnings

[brittytino]

Why

Fixes #3282.

CommonNameLoggingTrustManagerFactoryWrapper.NeverExpiringX509Certificate extends X509Certificate directly. getSubjectDN()/getIssuerDN() are abstract on that class (deprecated since JDK 16, but never made non-abstract or removed), so any direct subclass is still required to implement them. The build currently emits 4 deprecation warnings from this file (one at each override declaration, one at each internal delegating call).

What / How

I looked at swapping the internal delegation to getSubjectX500Principal()/getIssuerX500Principal() as suggested in the issue, but that changes the runtime type of the returned Principal: the original certificate's getSubjectDN() typically returns an implementation-specific Principal (e.g. sun.security.x509.X500Name), while getSubjectX500Principal() returns a javax.security.auth.x500.X500Principal. I verified with a real self-signed cert that these two are not .equals() to each other even though their toString() output is identical:

subjectDN class: class sun.security.x509.X500Name
subjectX500 class: class javax.security.auth.x500.X500Principal
subjectDN.equals(subjectX500): false

Making that swap would silently change the wrapper's observable behavior and would break the existing test (CommonNameLoggingTrustManagerFactoryWrapperTest#testNeverExpiringX509Certificate), which asserts cert.getIssuerDN() / cert.getSubjectDN() equal wrappedCert.getIssuerDN() / wrappedCert.getSubjectDN().

Since the overrides can't be removed and changing their delegation isn't behavior-preserving, this PR keeps the delegation exactly as-is and adds @SuppressWarnings("deprecation") on the two required overrides (with a short comment explaining why they can't be removed), plus on the test method that intentionally exercises the deprecated methods to verify backward-compatible behavior. No functional change.

Testing

• ./gradlew :clients:compileJava :clients:compileTestJava — the 4 warnings from this file are gone; only pre-existing, unrelated warnings remain (AccessController/Subject.getSubject, JsonNode#fields(), ObjectMapper#setSerializationInclusion).
• ./gradlew :clients:test --tests "org.apache.kafka.common.security.ssl.CommonNameLoggingTrustManagerFactoryWrapperTest" — all 11 tests pass.
• ./gradlew --build-cache :clients:checkstyleMain :clients:checkstyleTest :clients:spotbugsMain :clients:spotbugsTest :clients:spotlessJavaCheck — all pass, no formatting changes.