make gitlab use cpp syntax highlighting for icpp files

Following the documentation here https://docs.gitlab.com/ee/user/project/highlighting.html
I added a line to .gitattributes to highlight icpp files as c++.

assigned to @clemenci

changed milestone to %v33r2

approved this merge request

enabled an automatic merge when the pipeline for 86728a51 succeeds

@clemenci no idea why this one test failed, but I doubt it had anything to do with my changes :D

It's a time out... just click retry on that job.

merged

mentioned in commit 6b2c274a