Remove server OTel instrumentation from react-router

[chargome]

Now that the instrumentation API is the default and the minimum React Router version is 7.15, remove the server-side OTel instrumentation from @sentry/react-router:

• Delete the InstrumentationBase proxy (server/instrumentation/reactRouter.ts) that patches createRequestHandler.
• Remove the OTel data-loader span-creation path and its Node-version gate in server/integration/reactRouterServer.ts, plus the related flag in server/serverGlobals.ts.
• Drop the bogus * route cleanup in processEvent/processSegmentSpan that only exists for the OTel/express path.

Part of #22290.

[linear-code]

JS-3125