(maint) Update puppet-agent branch to `main`
(maint) Update puppet-agent branch to `main`

Previously, the puppet-agent branch for Platform 7 was called "next",
but it was recently renamed to "main". This commit updates our tracking
branch to point to `main`.

puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppetserver