Update license header

[albestro]

This PR should aim at either

• updating license header for this year
• or fixing it once a for all as other open source projects are doing recently (e.g. Remove years from license headers spack/spack#48352).

EDIT: We opted for the second option, i.e. removing completely the year information.

In addition to the license header update, it is worth highlighting another change which is drowned in the multitude of small changes. Some files were not checked for the license header, more specifically:

• *.yml files (*.yaml were already checked)
• Makefile (we have one for building documentation)
• scripts without extensions (we have some in ci/, limited just to that folder for simplicity)

see 3c88c96 and 8d8a208

[RMeli]

+1 for the second option.