Change the GAV and package names for junit5 extensions to junit-jupiter. Provide OpenRewrite recipe for migration

[manovotn]

Fixes #293

Having junit5 in the name wasn't the best choice; this PR follows up on the issue regarding JUnit 6 and will allow us to rename the artifact and packages to be JUnit agnostic, only relating to the jupiter engine it uses.
We can then also use JUnit 6 as baseline (we are keeping 5.x ATM to not confuse users although we can run with both just fine).

Artifact ID changed from junit5 suffix to junit-jupiter.
junit5 packages were changed to junit.jupiter.

The OpenRewrite recipe covers both of these changes and currently hardcodes 6.0.0.Final version which would be the next version for CDI 5/Weld7.
I have tried the recipe on a dummy project and that seems to work fine.
One thing it won't cover are custom service providers (custom enrichers) but those should be rare enough and simple to fix.

[manovotn]

I have added separate commits that:

• Upgrade JUnit version to 6 baseline and drop the CI testing for v5
• Update Weld to 7.x (hence CDI 5.0) although these are of course Alpha releases for now
• Update project version to 6.0.0-SNAPSHOT
  • I have also created a 5.0 branch from current main in case we need fixes going only there