Add known Kestrel connection error types

[JamesNK]

Changes

.NET 9 adds connection error reasons to Kestrel connection duration metric. This PR updates the spec with the known types.

Preview: https://github.com/JamesNK/semantic-conventions/blob/jamesnk/kestrel-error-reason/docs/dotnet/dotnet-kestrel-metrics.md#metric-kestrelconnectionduration (scroll down to error.type table)

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.

[JamesNK]

All done.

There is a warning that a link the markdown - https://blogs.oracle.com/linux/post/understanding-linux-kernel-memory-statistics - is dead when it's not.

[lmolkova]

thank you!