spec: maybe always use MUST and not SHALL?

[epoberezkin]

Although SHALL is defined as an synonym to MUST I always confuse it with SHOULD that means a different thing.

This line, e.g.: https://github.com/json-schema-org/json-schema-spec/blob/master/jsonschema-core.xml#L289

I was about to ask why ignoring other keywords when $ref is used became optional only to realise it didn't...

Just an idea, not that important really...

[awwright]

Yeah, I can see that.

I think I wrote that... Yeah, yeah I did.

(I'm wondering why I wrote that because I made an effort to replace a few instances of SHALL with MUST)