[Build] Stop tracking flatten-maven-plugin generated .flattened-pom.xml files

[shangeyao]

Problem

#4426 introduced Maven CI-friendly ${revision} versioning with flatten-maven-plugin, but also committed 10 .flattened-pom.xml files into the repository.

These files are build artifacts generated during process-resources (like target/). They should not be version-controlled.

Apache SeaTunnel uses the same ${revision} + flatten-maven-plugin approach and explicitly gitignores them:

*.flattened-pom.xml

Expected behavior

• Source pom.xml files keep ${revision} for centralized version management
• flatten-maven-plugin remains configured for install/deploy
• .flattened-pom.xml is generated locally during Maven builds but never committed

Proposed fix

1. Add *.flattened-pom.xml to .gitignore
2. Remove the 10 committed .flattened-pom.xml files from the repo

Related: #4426