Remove support for Node.js v16 after EOL

[timostamm]

Support for Node.js v16 ends on September 11th, 2023 (source).

We have not decided whether to drop support on our side right away. This issue tracks tasks to do once we do drop support:

• remove polyfills (node-headers-polyfill.ts, node16-polyfill-helper.spec.ts)
• remove special casing in H2 session manager (http2-session-manager.ts)
• remove special casing in tests (node-universal-client.spec.ts)
• re-add sideEffects: false removed in https://github.com/connectrpc/connect-es/pull/478/files for the headers polyfill