Change maintainers to be @open-telemetry/specs-semconv-maintainers

[jack-berg]

Now that semantic conventions have been split out from the opentelemetry-specification repo, and have their own approvers / maintainers, can we change the codeowners of this repo to be @open-telemetry/specs-semconv-maintainers?

[Oberon00]

This is a mixed repository which also contains other tooling (proto build).

[hughesjj]

Can we add instead of replace the codeowners? example from docs

[hughesjj]

Looks like we already have @open-telemetry/specs-semconv-approvers set for such.

Feel free to kill/merge/modify this PR depending if we still want this