add sha384 signature to manifest.xml

[alikon]

with the merge of joomla/joomla-cms#17619. in the 3.9 branch

Summary of Changes

added the <sha384></sha384> tag to the manifest file

Testing Instructions

on joomla < 3.9 nothing change
on joomla 3.9 the checksum is checked

[zero-24]

Why not use sha512? Or is there a special reason to use sha384?

[alikon]

no special reason except than composer use sha384 iirc.... and this one https://stackoverflow.com/questions/10061532/why-chose-sha512-over-sha384