docs: note external HTTP TLS preflight fix

Author: Ralph Küpper

changelog.d/8754-external-http-tls-preflight.md

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+Fixed optimized `node:net`, `node:http`, `node:https`, and `node:http2` builds
+after the TLS parity split. These external wrappers now retain Perry's shared
+TLS server and SNI/ALPN preflight provider without also linking the bundled
+network implementation, eliminating the `js_tls_client_preflight` undefined
+symbol that blocked HTTP gap and GC-stress fixtures at compile time.