[FLINK-40060][Connectors/JDBC] Remove redundant org.postgresql dependency in flink-connector-jdbc-backward-compatibility

[ocb3916]

What is the purpose of the change

flink-connector-jdbc-backward-compatibility/pom.xml declares the
org.postgresql:postgresql test dependency twice:

• once using the ${postgres.version} property
• once hardcoded as 42.7.3

Since ${postgres.version} already resolves to 42.7.3, the two
declarations are identical and the hardcoded one is redundant.

Brief change log

• Removed the duplicate org.postgresql:postgresql dependency that
  hardcoded the version as 42.7.3, keeping only the declaration that
  uses the ${postgres.version} property.

Verifying this change

This is a test-scope dependency cleanup with no production code
impact. Existing tests should continue to pass unchanged since the
remaining dependency resolves to the same version.

Does this pull request potentially affect one of the following parts

• Dependencies (test scope only): yes
• The public API, i.e., is any changed class annotated with
  @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths: no
• Anything that affects deployment or recovery: 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

[boring-cyborg]

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