Update sigtest-maven-plugin to 1.6, Update TCK ref guide for Arquillian EJB, misc. final 2.1.0 changes

[scottkurz]

A list of things in this PR:

1. Bump version to 2.1.0-SNAPSHOT
2. Bump up Weld + CDI deps to more recent versions
3. Move Arquillian instructions from README.md to merge into TCK ref guide
4. Expand existing TCK ref guide sections to incorporate new ejb/web executions
5. Fix javadoc errors by upgrading plugin, excluding test classes from javadoc gen
6. Add logging config for tck.exec module (disabled)
7. Upgrade sigtest plugin to 1.6
8. Require <packages>jakarta.batch.**</packages> config for sigtest execution, add to TCK guide
9. Convert EJB -> Enterprise Bean when externalized
10. Increase Open Liberty server stop timeout

[scottkurz]

@OndroMih I still have a list of things to adjust, e.g. the "Expected Output". But I wanted to get this out there to show basically how I'm thinking of organizing the TCK reference guide.

Essentially I moved all the jakarta.batch.arquillian.exec-parent/README.md into the guide .adoc, to avoid confusion about which was the official doc. There were a lot of editorial decisions in merging that info.. I didn't quite get into test.properties completely.

Well, it could always be changed for the next few days. Hopefully nothing here looks too bad to you.

Will keep working on this today.

[scottkurz]

I'm going to merge this to keep the release moving. We can still review during the release review.