(No longer) Compiling against Java 8

Hello Tapestry developers,

In the main development branch we are switching to Jakarta EE. (We’ll 
continue to support Java EE in the `javax` branch.)

As I see it, switching to Jakarta EE implies we need to give up 
compiling against Java 8 (whilst keep compiling against Java 11 and Java 
17).

Consider just one example why:
1. `tapestry-runner/…/JettyRunner.java` depends on Jetty.
2. Jetty 11+ is required for Jakarta EE support.
3. Jetty 11 requires Java 11+

Unless anyone objects, I would go ahead and remove Java 8 from 
`tapestry-5/.github/workflows/build-pull-request.yaml`.

Thank you,

Volker