Migrate PushBuilder to 103 in ExternalContextImpl

[BalusC]

To satisfy jakartaee/faces#2173

[BalusC]

Actual HttpServletResponse#sendEarlyHints() impl is currently still not available in GlassFish 9 (Response has only a TODO comment) and Tomcat 12 is not yet available, even not as a beta (which is rather unexpected as they were usually early at it). Jetty 12 appears to have a native impl for 103 but still no Servlet 6.2 interface impl yet.

So this means I won't be able to actually test the HttpServletResponse#sendEarlyHints() at the time of writing.

[BalusC]

And building Mojarra with a hard Servlet 6.2-introduced dependency makes it unusable on currently available 6.1 containers as it would throw NoSuchMethodError on HttpServletResponse#sendEarlyHints() when just opening webpage with resources. So we need a middle ground for now until 6.2 containers are available and mainstream. Until then we still need 6.1 containers (Tomcat 11 especially) in order to be able to run/test Mojarra-5.0-in-development at all.