Update semantic-conventions to 1.21.0, publish all schemas, drop draft status, enable auto-update

[chalin]

• Contributes to Adjust to move Semantic Conventions out of OTel spec #2721
• Updates semconv submodule to 1.21.0
• Enabled the publication of schema for 1.21.0
• Removes draft status from semconv pages -- SEMCONV PAGES WILL APPEAR ON THE PRODUCTION SERVER
• Enabled auto-update of the semconv version
• Note that the OTel spec is at 1.23.0, so links into the OTel spec at 1.22.0 appear as external links; they do not link to the corresponding OTel website spec page

Preview: https://deploy-preview-3033--opentelemetry.netlify.app/docs/specs/semconv

/cc @jsuereth @joaopgrassi

[joaopgrassi]

Does Rename pages under /docs/general/ to avoid general in the page name, etc impact in anything after we release this version to production? I guess the URLs right, but that can be fixed with the aliases?

[chalin]

@joaopgrassi - we can do it in either order. Fixing the names first is a bit easier in that we don't need to add aliases.

If you and @jsuereth prefer, I can leave the senconv pages in draft status. Let me know.

[jsuereth]

I think publishing as-is is fine. We can continue to refine/update pages as we go.