Release new version for agda 2.7

[felixwellen]

Agda 2.7 was released and we should release a new version of the cubical library, following the intructions here:

https://github.com/agda/cubical/blob/master/RELEASE.md

I recently used the library with the current master of agda, so I expect there is nothing else to do and anyone with the rights can do a release (and anyone who can make a PR can do the changes). I'll do this at some point after my vacation.