renderRfc1123DateTimeString can be faster

[He-Pin]

Motivation:

In recent zio/zio-http#3550 , it shows much faster than the default one, I think this can be faster than the renderRfc1123DateTimeString in current pekko-http too.

[jrudolph]

Is that based on any real world measurements? You only need to render once per second for http, so shaving off 500ns every second does not matter. Besides that DateTime rendering is probably just fast enough since its optimization in 2011. Not sure if we tweeted about it, though...

[987Nabil]

This is a synthetic benchmark for sure. The goal is actually not performance, but decoupling from netty.
The benchmark was only to show that we did not become slower.
If you should make a change, idk. But I did my best to avoid any unnecessary operations/allocations.