Fix: prod deploy workflow uses infra release tag as JAR version

[jcpitre]

Problem

When a release is published on this repo, the release trigger passes github.event.release.tag_name (e.g. v1.0.0) as the GBFS_API_IMAGE_VERSION. The deployer then uses that value as the Maven artifact version, causing the build to fail because no such version exists in Maven Central:

Could not find artifact org.mobilitydata:gbfs-validator-java-api:jar:v1.0.0 in central

Root cause

The infra repo now has its own versioning (v1.0.0) that is decoupled from the gbfs-validator-java JAR version.

Fix

Remove github.event.release.tag_name from GBFS_API_IMAGE_VERSION. When triggered by a release event (or a workflow_dispatch with no version specified), the field is now empty and the deployer correctly falls back to RELEASE, downloading the latest release from Maven Central.

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