Remove copies of metric-requirement-level and attribute-requirement-level from semconv repository.

[jsuereth]

See: #5 (comment)

TL;DR: we are currently soft-linking to requirement levels for attributes/metrics defined by the specification.

• These should become deep links to the specification
• We need more automation around updating the specification version
• The version semconv relies on (1.21) has not been released yet.

This is an issue to remind us to clean this up once semconv 1.21 is released and we can link to stable versions.

Related see #9

[jsuereth]

Note: Here is the error message when trying to link against 1.20 of specification:

  [✖] https://github.com/open-telemetry/opentelemetry-specification/tree/1.20.0/specification/common/attribute-naming.md → Status: 404
  [✖] https://github.com/open-telemetry/opentelemetry-specification/tree/1.20.0/specification/common/attribute-requirement-level.md → Status: 404
  [✖] https://github.com/open-telemetry/opentelemetry-specification/tree/1.20.0/specification/metrics/metric-requirement-level.md → Status: 404