Prepare Maven Central release for FlexTrack 1.2.0

[alirezat66]

Summary

• publish FlexTrack as io.github.alirezat66:flextrack:1.2.0 through Maven Central
• add signed Central Portal publishing with automatic release
• remove the GitHub Packages token requirement from consumer setup
• add complete POM metadata, sources, javadoc, changelog, and publisher documentation
• keep ordinary CI independent from release signing secrets and validate publication artifacts

Verification

• ./gradlew :flextrack:test :flextrack:lint :flextrack:assembleRelease
• generated and inspected the Maven POM, AAR, sources JAR, javadoc JAR, and Gradle module metadata
• release workflow fails early when any Central/signing secret is absent

Release notes

After merging, configure the four repository secrets documented in docs/maven-central-release.md, then create tag v1.2.0. The tag workflow will sign, upload, and automatically release the package through the Central Portal.