http2: fix excessive CPU usage when using allowHTTP1=true

When Http2SecureServer is configured with `allowHTTP1=true`, it calls
`setupConnectionsTracking` to start monitoring for idle HTTP1
connections. `setupConnectionsTracking` expects to have
`this.connectionsCheckingInterval` property defined, but it does not
exist on `Http2SecureServer`. This `undefined` value is passed to
`setInterval`, which results in `checkConnections` being called on
every tick, creating significant additional load on the server CPU.
The fix is to define `this.connectionsCheckingInterval` on the
Http2SecureServer instance.

Refs: nodejs#51569

lib/internal/http2/core.js

@@ -3185,6 +3185,7 @@ class Http2SecureServer extends TLSServer {
     if (options.allowHTTP1 === true) {
       this.headersTimeout = 60_000; // Minimum between 60 seconds or requestTimeout
       this.requestTimeout = 300_000; // 5 minutes
+      this.connectionsCheckingInterval = 30_000; // 30 seconds
       this.on('listening', setupConnectionsTracking);
     }
     if (typeof requestListener === 'function')