Upgrade guide outdated

[nikolaushuber]

The upgrade-guide.md seems to be pretty outdated. Maybe we should at least mention that at the start of the file?

[raehik]

Yep. It was originally written to make upgrading between fortran-src versions even easier for any end users while we tinkered with syntax representation. But I think our main end user remains ourselves (CamFort). I don't think it's critical to keep up to date. Useful place to discuss changes on a higher level though.