[chore][connector/servicegraphconnector] Updating semconv version

[anarwal]

Description

1. Removed 3 old imports: v1.25.0, v1.28.0, v1.38.0
2. Added single import: conventions "go.opentelemetry.io/otel/semconv/v1.40.0"
3. PeerServiceKey now comes from v1.40.0 (symbol exists there)
4. DBNameKey ("db.name") → string literal (deprecated, no v1.40.0 equivalent)
5. DBSystemKey ("db.system") → string literal (renamed to db.system.name in v1.40.0, but we keep reading the old key)

Link to tracking issue

Fixes connector/servicegraphconnector from #22095

Testing

Ran go mod tidy && go build ./... && go test ./... from connector/servicegraphconnector

Authorship

• I, a human, wrote this pull request description myself.