Use GenAI instead of LLM and other clean ups

[lmolkova]

Fix #969
With #811 merged, we now have inconsistent LLM vs GenAI usage. This PR:

• uses GenAI instead of LLM
• cleans up unused properties in yaml

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[drewby]

LGTM.

[codefromthecrypt]

I'm trying to understand this change, particularly it came from @drewby who had this at the end of the issue

For attributes that are only relevant to LLM, the reference can remain.

What I noticed was that LLM was pretty much if not completely stripped from the docs. In most cases replacing it with GenAI or GenAI model.

Assuming the wildcard is correct (there's no attribute that is only about LLM and not also GenAI in general), it may still help to introduce the topic a bit better on the span or metric docs.

for example, change "A request to an Generative AI is modeled as a span in a trace." to "A request to a Generative AI system is modeled as a span in a trace. For example, this could be an LLM or image generation prompt"

I'm not sure the exact language, but hopefully this gets the point across. I like generalizing, but also it would be nice to know if anything was accidentally wildcarded from LLM to GenAI and also if we can keep the word "LLM" somewhere to help contextualize the spec.

I'm happy to raise a PR if folks agree on what should happen here.