Publish a validator release

[isabelle-dr]

It's time for a new validator release!

To do:

• publish the release following the steps in RELEASE.md
• outreach to producers that are affected (Emma)
• send the release e-mail to users that asked to be informed via e-mail (Emma)

[davidgamez]

It will be nice to have #1487 done before the release.

[jcpitre]

This step:
4. Remove all sha Docker images added since last release
has to be done by someone with the proper permission.

[jcpitre]

Tested the sonatype repo by replacing the local dependencies implementation project(':main') by implementation 'org.mobilitydata.gtfs-validator:gtfs-validator-main:4.2.0' for core, main and model.
All tests passed.

[jcpitre]

Tested the windows and mac installers. They work.

[jcpitre]

Went from pre-release to release in https://github.com/MobilityData/gtfs-validator/releases/tag/v4.2.0 and made it latest.
Set the version to 4.2.0 in the wiki page https://github.com/MobilityData/gtfs-validator/wiki/Current-Version
@emmambd Users of the desktop version will be notified of a new version from now on.

[emmambd]

Completed - producers affected will be contacted after minor release including #1613