use github_branch: main for galaxy import
use github_branch: main for galaxy import

Galaxy imports of the network role are failing:
https://galaxy.ansible.com/my-imports/1710
```
Task "703539" failed: Command '['git', 'clone', '--quiet', '--depth', '1', '--branch', 'master', 'https://github.com/linux-system-roles/network.git', '/var/tmp/galaxy/imports/tmpnxdxwzmg']' returned non-zero exit status 128.
```
We need to tell galaxy to use `main` instead of `master`.  The galaxy
roles documentation says that `github_branch` may be used for this
purpose:
https://galaxy.ansible.com/docs/contributing/creating_role.html#role-metadata

Signed-off-by: Rich Megginson <eddfb389174a82ceee7387bfddd9793f5b4725f4@redhat.com>

linux-system-roles/network,linux-system-roles/network,linux-system-roles/network