Switching to Jena5 for development

[Andy Seaborne]

With the release of Jena 4.10.0, we can switch branch "main" to Jena5 
for development.

There'll be a branch "jena4", starting at the commit for the CHANGES update.
Then ...

  One last rebase of "main" into "jena5" and force push of "jena5"
  Merge (fast-forward) jena5 to main.
  Remove branch jena5.

    Andy