[FLINK-40090][docs] Modernize cryptography export notice in distribution README

[MartijnVisser]

What is the purpose of the change

Adhering to the requirements outlined in https://infra.apache.org/crypto.html#inform

Brief change log

• Updated README.txt that gets included in the distribution

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

[flinkbot]

CI report:

• 039d258 Azure: FAILURE
• 8214042 Azure: PENDING

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