Our license isn't compatible with our deps' ones

[Al2Klimov]

Describe the bug

As https://github.com/Icinga/icingadb/pull/379/checks?check_run_id=3745669016 reports, right now we're violating our own license by combining the GPLv2 app with a partially Apache2.0 lib.

To Reproduce

Run go mod vendor and read all the licenses in vendor/ (or just trust the linked check).

Expected behavior

Either

Explicitly upgrade our README.md and license headers (which license headers? ba dumm tss) GPLv2 -> GPLv2+. (Luckily as #380 (comment) reports all contributors have signed a CLA which allows sublicensing.)

Or

Get rid of some deps (at least gopkg.in/yaml.v3).

CC @N-o-X @julianbrost

[Al2Klimov]

PS: IMAO GPLv2+ == GPLv3- makes GPLv2+ + Apache2.0 working, but IANAL.