Upgrade to servlet 6 spec

[gkresic]

Migrate to servlet 6 spec. As of 17.02.2023. only Jetty doesn't support it. Version 12 (currently in alpha) will bring support.

For reference:

Jetty versions: https://www.eclipse.org/jetty/download.php#version-table Tomcat verions: https://tomcat.apache.org/whichversion.html#Apache_Tomcat_Versions Undertow versions: https://undertow.io/downloads.html -> doesn't cover newer versions (2.3 supports Servlet 6, as documented on GitHub: https://github.com/undertow-io/undertow)

Depends on #624