[FLINK-39375][API / Core] Add explicit serialVersionUID to WatermarkAlignmentParams

[Myracle]

What is the purpose of the change

This pull request adds an explicit serialVersionUID to WatermarkAlignmentParams to ensure serialization compatibility across different Flink versions. Without an explicit serialVersionUID, the JVM auto-generates one based on the class structure, which can change unexpectedly if the class is modified, leading to deserialization failures.

Brief change log

• Added private static final long serialVersionUID = 1L; to WatermarkAlignmentParams

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): yes (WatermarkAlignmentParams is annotated with @PublicEvolving)
• The serializers: yes (this change pins the serialVersionUID to ensure stable serialization compatibility)
• 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:

• 6a07705 Azure: SUCCESS

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