Bump main to 6.0.0~pre1

[azeey]

Part of gazebosim/gz-jetty#70

[scpeters]

windows CI needs a gazebodistro file for gz-cmake6.yaml

13:18:30 # BEGIN SECTION: get open robotics deps (gz-cmake6.yaml) sources into the workspace
13:18:31 Cloning into 'gazebodistro'...
...
13:18:31 * master
13:18:31 The system cannot find the file specified.

[scpeters]

windows CI needs a gazebodistro file for gz-cmake6.yaml

13:18:30 # BEGIN SECTION: get open robotics deps (gz-cmake6.yaml) sources into the workspace
13:18:31 Cloning into 'gazebodistro'...
...
13:18:31 * master
13:18:31 The system cannot find the file specified.

for jetty, this was done in gazebo-tooling/gazebodistro#182, though I would consider setting all the branches to main for the new yaml files