Bump Spark 2 branch to 2.4.7

[Fokko]

Spark 2.4.6 contains some incorrectness bug.

Related #1386

[RussellSpitzer]

Thanks for doing this, perhaps we should get one of those version update bots so we don't have to keep doing these manually?

[Fokko]

Yes, that's a good point. For now, I see two issues:

• Not sure if there is something of the self available for Gradle (using the versions.lock).
• Apache doesn't allow to have 3rd parties write access to the repository. Dependabot requires this :(

[RussellSpitzer]

Could we have dependabot just work off another repository and submit pr's? Then at least we only have to click merge :)

[Fokko]

I don't mind doing it now and then. It is a nice way to keep in touch with the project. Also, some other dependencies need some love, such as #1648

[rdblue]

Thanks for the update, @Fokko!