[chore] Mark dotnet-specific metrics and attributes stable in yaml to match markdown

[lmolkova]

Changes

Follow-up on #781 - .NET md documents are marked stable, but yaml metrics and attributes are not.
This marks dotnet-specific (with corresponding prefix) attributes and metrics as stable.

Merge requirement checklist

• 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.

[joaopgrassi]

ping @open-telemetry/semconv-dotnet-approver

[antonfirsov]

I assume dotnet-dns-metrics.md and dotnet-http-metrics.md are not considered to be stable? I defer to @JamesNK for the server-side bits.