[FLINK-33321][hotfix] VertexFlameGraphFactoryTest#verifyRecursively doesn't work on java 21

[snuyanzin]

What is the purpose of the change

looks like in the first PR #23556 there was a mistake, sorry for that
this PR is going to address it

Verifying this change

i 've set java21 for my pipeline and here the results
Current version of test
https://dev.azure.com/snuyanzin/flink/_build/results?buildId=2435&view=logs&j=9dc1b5dc-bcfa-5f83-eaa7-0cb181ddc267&t=511d2595-ec54-5ab7-86ce-92f328796f20&l=8087

and fixed version
Proof link https://dev.azure.com/snuyanzin/flink/_build/results?buildId=2436&view=logs&j=9dc1b5dc-bcfa-5f83-eaa7-0cb181ddc267&t=511d2595-ec54-5ab7-86ce-92f328796f20&l=76

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)

[snuyanzin]

@1996fanrui could I ask to review this since you've looked at the first version of it?

[1996fanrui]

Thanks for the fix!

LGTM.

[flinkbot]

CI report:

• 0d2a176 Azure: SUCCESS

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