fix: support devServer.server option, avoid deprecation warning

Closes #7024
Fixes #7118

I choose to not merge the PR because I don't want add additional ways
to configure https for dev server (`--http2` command line argument,
`process.env.HTTPS`, etc.)

In the current implementation, `spdy` can only be configured by setting
`{ devServer: server: { type: 'spdy' } }`.
This is a deliberate choice, because SPDY support in Node.js 15+ is
broken anyway. I don't want bother refactoring the old code to
accommodate this broken feature.

packages/@vue/cli-service/lib/commands/serve.js

@@ -199,7 +199,14 @@ module.exports = (api, options) => {
     }, projectDevServerOptions, {
       host,
       port,
-      https: useHttps,
+
+      server: {
+        type: protocol,
+        ...(typeof projectDevServerOptions.server === 'object'
+          ? projectDevServerOptions.server
+          : {})
+      },
+
       proxy: proxySettings,
 
       static: {