Use SHOULD instead of MUST for span kinds on DB, GenAI and FaaS

[lmolkova]

Fixes #1315

Changes

Relaxes DB, GenAI and FaaS span kind conventions to use SHOULD instead of MUST.
A good justification for this would be in-memory db or gen-ai model running in the same process.

Since there are a lot of variations of FaaS spans and they usually follow some other conventions (HTTP or messaging) their span kinds can't be too strict.

Merge requirement checklist

• CONTRIBUTING.md](https://github.com/open-telemetry/semantic-conventions/blob/main/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.

[codefromthecrypt]

post-merge approve. I completely agree that in-process (possibly also IPC) being able to mark themselves INTERNAL is a step forward!