Change normativity keywords to ISO conventions

[arnoweiss]

The spec currently leans in the direction of RFC 2119 but should in the long run rather follow ISO conventions (see chapter 7). Someone needs to go over all normative statements and select the right keywords.

[anilturkmayali]

We may plan a workshop (half-day?) for this.

[juliapampus]

The document does not have to stick to ISO conventions. I will check the document for consistency (and open a PR).