[collectd] Changing collectd's main branch.

[Florian Forster]

Hi all,

I have updated all pull requests to use "main" as their base branch 
instead of "master", i.e. the branch they target for merging into. Pull 
requests with other base branches were not modified.

Best regards,
—octo
-- 
collectd – The system statistics collection daemon
Website: http://collectd.org
GitHub:  https://github.com/collectd
Twitter: http://twitter.com/collectd