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).

[cka-y]

Tasks:

• Update the bindings
• Release package

[jcpitre]

@richfab @testower It looks like the current validator stops at v3.0. See VersionFactory.java
Since RC and RC2 were not implemented, do we need to implement RC3? Or should we wait for a non-RC version?

[richfab]

@jcpitre I highly recommend that we support RC versions in the validator.
Indeed, it helps First Adopters to test their new feed. That's currently the case with the Javascript validator.
I'll let @emmambd confirm the decision.

[emmambd]

@richfab @jcpitre Confirmed - it's a part of the release process and we should proceed. The Java validator is only recently under our stewardship so this discrepancy makes some sense.