ZOOKEEPER-4993: Lower "Attempting to delete candidate container..." logging to DEBUG

[Pyolar]

This change lowers the candidate container deletion log from INFO to DEBUG to avoid excessive log volume while preserving the constructor configuration log at INFO.

A regression test verifies that the candidate deletion message is emitted at DEBUG level.

Jira: https://issues.apache.org/jira/browse/ZOOKEEPER-4993

Tests:

• mvn -pl zookeeper-server -am -Dtest=CreateContainerTest -Dsurefire.failIfNoSpecifiedTests=false test
• mvn -pl zookeeper-server -am -DskipTests compile checkstyle:check spotbugs:check
• mvn verify spotbugs:check checkstyle:check -Pfull-build -Dsurefire-forkcount=4 reached 3,239 tests. It reported 7 unrelated OCSP/SSL failures in ClientSSLRevocationTest and QuorumSSLTest; all CreateContainerTest tests passed.

[Pyolar]

/retest

[Pyolar]

Jenkins build #1 reached compilation and failed only in two unrelated tests: ServerCnxnFactoryTest.testShedConnections_SmallPercentageRoundsToZero{FactoryType}[2] (probabilistic 1% shedding returned 1 instead of the test expected 0) and QuorumRestartTest.testRestartingLeaderMultipleTimes (30-second leader-election timeout). The PR-specific CreateContainerTest passed in that run. Focused local validation also passes on this commit: ServerCnxnFactoryTest 10/10, QuorumRestartTest 3/3, and CreateContainerTest 14/14. No source changes are needed for PR #2430; please rerun the Jenkins check.

[anmolnar]

You don't need to write unit test to cover a log level change.

[Pyolar]

@anmolnar Removed the unit test as suggested. The PR now only contains the one-line log level change in ContainerManager.java.