Do not host download artifacts on maven central

[chibenwa]

I received the following notice:

Sorry, but the announce cannot be accepted.
There are some some errors on the download page.

Sources for releases must be downloaded from the ASF mirror system.
The links for the mailet 3.6.0 sources currently point to the Maven repo.
They must use the mirror system via closer.lua and the associated sigs and
hashes must be served from downloads.apache.org/james...

Please fix the download page, check, and submit a fresh announce.
Thanks!

Sebb