Update Checkstyle version from 8.39 to 10.7.0

[ljacqu]

We currently run Checkstyle in CodeClimate, meaning the version of the Checkstyle rules needs to be in sync with what CodeClimate uses. It's not updated frequently, and now IntelliJ doesn't even have such old versions of Checkstyle.

Goal of this issue is to update to a newer version of Checkstyle by using another "channel" in CodeClimate:
https://github.com/codeclimate/codeclimate-checkstyle/tree/channel/checkstyle-10-7-0

It should be possible to specify the channel in .codeclimate.yml by adding a property channel: "checkstyle-10-7-0" to the Checkstyle check (search for "beta" in https://docs.codeclimate.com/docs/checkstyle to see how)

To do

• Change channel to checkstyle-10-7-0 in .codeclimate.yml for the Checkstyle plugin
• Migrate .checkstyle.yml to the proper format for Checkstyle 10.7.0
• Update the comment in .checkstyle.yml to not mention 8.39 anymore

[ljacqu]

For what it's worth, the long-term goal is probably to ditch CodeClimate if there's a suitable alternative for Sonar/Checkstyle, cf. #376