feat(node)!: Drop support for Fastify <3.21.0 (#22752)

## What

Drops support for Fastify `<3.21.0` by removing
`FastifyInstrumentationV3` (and `instrumentFastifyV3`). The node
`fastifyIntegration` becomes a thin pass-through to the channel-based
integration in `@sentry/server-utils`.

## Why

The channel-based `fastifyIntegration` (default since #22501) covers
Fastify `>=3.21.0 <6`. The v3 OpenTelemetry instrumentation only existed
to fill the `>=3.0.0 <3.21.0` gap, and it was the last remaining
import-in-the-middle module-patcher in `@sentry/node`. Dropping the old
Fastify range lets us remove it.

Ref: #22346

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: andreiborza

dev-packages/e2e-tests/test-applications/node-fastify-3/package.json

@@ -10,7 +10,6 @@
     "clean": "npx rimraf node_modules pnpm-lock.yaml",
     "typecheck": "tsc",
     "test:build": "pnpm install && pnpm run typecheck",
-    "test:build-3_20": "pnpm install && pnpm add fastify@~3.20.0 && pnpm run typecheck",
     "test:assert": "pnpm test && pnpm test:override"
   },
   "dependencies": {
@@ -26,13 +25,5 @@
   },
   "volta": {
     "extends": "../../package.json"
-  },
-  "sentryTest": {
-    "variants": [
-      {
-        "build-command": "pnpm test:build-3_20",
-        "label": "fastify v3.20.0"
-      }
-    ]
   }
 }

dev-packages/e2e-tests/test-applications/node-fastify-3/tests/transactions.test.ts

@@ -98,7 +98,6 @@ test.skip('Sends an API route transaction', async ({ baseURL }) => {
     data: expect.objectContaining({
       'sentry.origin': 'auto.http.otel.fastify',
       'sentry.op': 'request_handler.fastify',
-      // format is slightly different in v3.20.0 and v3.21.0
       'fastify.type': expect.stringMatching(/request[-_]handler/),
       'http.route': '/test-transaction',
     }),

packages/node/src/integrations/tracing/fastify/index.ts

@@ -1,8 +1,6 @@
-import type { Integration, IntegrationFn } from '@sentry/core';
-import { defineIntegration, extendIntegration, getClient } from '@sentry/core';
-import { generateInstrumentOnce } from '../../../otel/instrument';
+import type { Integration } from '@sentry/core';
+import { defineIntegration, getClient } from '@sentry/core';
 import type { FastifyInstance, FastifyMinimal, FastifyReply, FastifyRequest } from './types';
-import { FastifyInstrumentationV3 } from './v3/instrumentation';
 import {
   fastifyIntegration as serverUtilsFastifyIntegration,
   instrumentFastify,
@@ -94,11 +92,6 @@ interface FastifyHandlerOptions {
 
 const INTEGRATION_NAME = 'Fastify' as const;
 
-export const instrumentFastifyV3 = generateInstrumentOnce(
-  `${INTEGRATION_NAME}.v3`,
-  () => new FastifyInstrumentationV3(),
-);
-
 function getFastifyIntegration(): FastifyIntegration | undefined {
   const client = getClient();
   if (!client) {
@@ -108,16 +101,6 @@ function getFastifyIntegration(): FastifyIntegration | undefined {
   }
 }
 
-const _fastifyIntegration = ((options: Partial<FastifyIntegrationOptions>) => {
-  const parentIntegration = serverUtilsFastifyIntegration(options) as FastifyIntegration;
-
-  return extendIntegration(parentIntegration, {
-    setupOnce() {
-      instrumentFastifyV3();
-    },
-  });
-}) satisfies IntegrationFn;
-
 /**
  * Adds Sentry tracing instrumentation for [Fastify](https://fastify.dev/).
  *
@@ -135,7 +118,7 @@ const _fastifyIntegration = ((options: Partial<FastifyIntegrationOptions>) => {
  * ```
  */
 export const fastifyIntegration = defineIntegration((options: Partial<FastifyIntegrationOptions> = {}) =>
-  _fastifyIntegration(options),
+  serverUtilsFastifyIntegration(options),
 );
 
 /**