Raise minimum supported fastify version to 3.21.0 #22658

Raise the minimum supported fastify version to 3.21.0 in v11.

The channel-based Fastify integration in @sentry/server-utils covers fastify >=3.21.0 <6. Support for >=3.0.0 <3.21.0 currently comes from the vendored OTel FastifyInstrumentationV3 (instrumentFastifyV3) that @sentry/node's fastifyIntegration layers on top. Dropping the older range lets us remove that vendored instrumentation entirely.

Needs a changelog/migration note since this is a breaking support-matrix change.

Removal details are tracked in a comment on #22346.