chore: update migration guide to reflect removal of deprecated CLI options

Author: bjohansebas

migration-v6.md

@@ -51,3 +51,24 @@ This document serves as a migration guide for `webpack-dev-server@6.0.0`.
   ```js
   const ip = Server.findIp("v4", true);
   ```
+
+- The following CLI options were deprecated and have now been removed. Please use the recommended alternatives:
+  - http2 -> server
+  - https -> server
+  - https-ca -> server.options.ca
+  - https-ca-reset -> server.options.ca
+  - https-cacert -> server.options.ca
+  - https-cert -> server.options.cert
+  - https-cert-reset -> server.options.cert
+  - https-crl -> server.options.crl
+  - https-crl-reset -> server.options.crl
+  - https-key -> server.options.key
+  - https-key-reset -> server.options.key
+  - https-passphrase -> server.options.passphrase
+  - https-pfx -> server.options.pfx
+  - https-pfx-reset -> server.options.pfx
+  - https-request-cert -> server.options.requestCert
+  - open-app -> --open-app-name
+  - server-options-cacert -> server.options.ca
+  - server-options-cacert-reset -> server.options.ca
+  - web-socket-server -> web-socket-server-type