Update to 6.19.1

Release notes:
https://puppet.com/docs/puppet/6.19/release_notes_puppet.html#release_notes_puppet_x-19-0

While here, add devel/rubygem-concurrent-ruby as a build dependency to fix
build with portmaster [1].

With hat: puppet
PR: 250480 [1]
Reported by: moritz.kraus@makandra.de [1]