Rename <student.version> to <grader.version>

[DavidWhitlock]

Ugh. I didn't catch it at the time, but when I refactored the version of the grader artifact that the project depend on, I named the variable in the top-level pom.xml student.version. It should have been grader.version. Duh.

But I've already deployed the release artifacts for Summer 2022. I'll fix this for next time.

[DavidWhitlock]

When I change the top-level pom.xml, I should also remove the reference to the JBoss bintray Maven repository. It doesn't exist anymore and, so, we don't need it.