node-core: remove legacy HTTP span hooks, preload workaround & re-evaluate keepAlive

[linear-code]

Cleanups in @sentry/node-core HTTP handling that are gated on v11 / OTel-decoupling.

TODOs:

• packages/node-core/src/integrations/http/httpServerSpansIntegration.ts:175 — remove the three legacy hooks, only onSpanCreated should remain
• packages/node-core/src/integrations/http/SentryHttpInstrumentation.ts:210 — likely removable once preload support is dropped
• packages/node-core/src/transports/http.ts:73 — evaluate whether keepAlive can be set to true (requires memory-leak testing on older Node)

[linear-code]

JS-3040