Replace system.network.state with network.connection.state

[ChrsMark]

Fixes #308

Changes

ref: #308 (comment)

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.

[ChrsMark]

@AlexanderWert @open-telemetry/specs-semconv-maintainers that should be good to go?

[lmolkova]

Looks good, left a few suggestions to clean up old enum members

[ChrsMark]

@lmolkova @open-telemetry/semconv-system-approvers I have tuned the PR so as the state values to be aligned with rfc9293.

I'm not sure if we should explicitly deprecate the enum values that were changed or removed entirely.

[lmolkova]

LGTM and thank you!

[lmolkova]

tagging @open-telemetry/semconv-security-approvers in case they want to take a look

[trisch-me]

@ChrsMark I think one test is failing, otherwise good to go!