consider adding @fastify/otel instrumentation

- Soonish instr-fastify will no longer be getting new versions (and will be removed from auto-instrumentations-node). We should drop it.
- Consider adding `@fastify/otel`, the blessed community replacement.
- We could have the new one get enabled via explicit `fastify` in `OTEL_NODE_ENABLED_INSTRUMENTATIONS`, or require explicit clear `@fastify/otel` name. I lean towards the latter for clarity. Having "fastify" in there "just work" migh be misleading because the instr is different.
- If we add `@fastify/otel` are there going to be dep duplicates to worry about? E.g. otel/instr version. We could perhaps use overrides... or just engage upstream.
- At time of writing it has the latest `@opentelemetry/instrumentation` on main, but hasn't had recent releases.

Start by tracing how instr-fastify is currently registered and how OTEL_NODE_ENABLED_INSTRUMENTATIONS selects instrumentation. Check the package dependency versions and upstream release status for @fastify/otel; done means the old instrumentation is removed and the chosen explicit-name behavior works without unresolved duplicate dependencies.

Written by the indexing model from the issue text.