Rename "Working group" (WG) to SIG everywhere

[trask]

The distinction between working group and SIG has been confusing to everyone, and so the GC has decided to do away with the term "working group" (WG) and use only the term SIG.

[svrnm]

We also have some slack channels that are named "-wg":

#otel-semantic-conventions-wg #otel-prometheus-wg #otel-agentmanwg #otel-system-metrics-wg #otel-llm-semconv-wg #otel-jvm-metrics-stability-wg #otel-wg #otel-db-client-semconv-stability-wg