chore: flatten pom, add licences to jar

[toddbaert]

Published pom hygiene improvements, and fix missing licence in jars:

• use flatten-maven-plugin and maven-resources-plugin
• in-lines elements from parent
• removes test/publish deps from published pom
• packages license into jar

Fixes: #1784

[gemini-code-assist]

Code Review

This pull request updates the project configuration by ignoring .flattened-pom.xml in .gitignore, marking compile-time dependencies (lombok and spotbugs) as optional, and adding plugins to flatten the deployed POM and bundle the root LICENSE file into published JARs. The review feedback highlights two issues: first, using ${maven.multiModuleProjectDirectory} requires a .mvn directory at the root of the repository to resolve correctly when building from submodules; second, the maven-resources-plugin will execute unnecessarily for the parent POM (which has pom packaging), creating an empty target/classes/META-INF directory, and should be configured to skip execution on the parent.

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[Kielek]

@toddbaert, is there any chance to make a release after this changes? It will be great to make the legal scanning happy.