Remove nodejs12

[BraisVQ]

Closes #936
Related to #962

[cschweikert]

I found another reference to NodeJS 12 in the docs: https://github.com/opendevstack/ods-quickstarters/blob/master/docs/modules/quickstarters/nav.adoc?plain=1#L24

And maybe there is more. I found this for example: https://github.com/opendevstack/ods-core/blob/0e02f3466cdd6f6e80a2c3a7d45509ff72f0ca97/tests/ods-verify/verify-ods-namespace-images_test.go#L24
That one is funny, because it misses any other node version.

[BraisVQ]

Updated the docs in the quickstarter repository.
The tests on core need some work true. Next year we want to use the golden tests on every quickstarter in order to do the testing internally so this will not be abandoned.