Update the Java validator to v3.1-RC3

[richfab]

Preparing the validator to support the new version:

1. Create a new branch release/vX.X-RC.
2. Add the new JSON Schema from the newly opened PR to gbfs-validator/versions/schemas.
3. Support validation of feeds using the new version.
4. Create a PR to merge release/vX.X-RC into master but don’t merge it for now.

Specific to this release: Remove the support of the previous version (v3.1-RC2) to keep only one RC version (if applicable).