[FLINK-38369][runtime] Fix flaky RestClientTest

[mateczagany]

What is the purpose of the change

RestClientTest has failed a few times in the past with exception Network is unreachable: /240.0.0.0:80 instead of connection timeout. By changing the test IP to an address of TEST-NET-1, which is reserved for "Documentation", hopefully we will have more reliable test results than using 240.0.0.0, which has undefined behavior on some network devices.

Brief change log

• Update unroutableIp to 192.0.2.1 which is part of TEST-NET-1 in RFC 5737

Verifying this change

Could not reproduce the issue locally in either way, so I did not have a reliable way to verify this.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• 0314e06 Azure: SUCCESS

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