Remove schema-next file and requirements to update it

[lmolkova]

Removes requirement to update schema-next.yaml when making breaking changes.

See:

• 1595 - we had a typo and there was no tooling to catch it. Since schema files are immutable, there is no way to fix it now

• 911 - we renamed attributes only on spans (but attributes apply to metrics too) and now there is no way to fix it

• many breaking changes can't be expressed:
  • https://github.com/open-telemetry/opentelemetry-specification/issues/3729
  • https://github.com/open-telemetry/opentelemetry-specification/issues/3992
  • https://github.com/open-telemetry/opentelemetry-specification/issues/3162
• the schema processor has been in noop state for the past 2+ years https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/75ceb5598807a8bb5c45d4d4f734ce2f6c88d25c/processor/schemaprocessor/transformer.go#L43
• https://github.com/open-telemetry/opentelemetry-specification/issues/3513
• https://github.com/open-telemetry/semantic-conventions/issues/617

Let's admit that the process is not working and stop updating schema files manually and invest into automated process - https://github.com/open-telemetry/semantic-conventions/issues/1426 and https://github.com/open-telemetry/weaver/issues/186

Merge requirement checklist

• [x] 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]