Permit to open discussions on release

[Danil-Grigorev]

Following https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/controlling-permissions-for-github_token#overview, we need to enable discussions: write on the job to allow gh execution to pass.

Context: https://github.com/rancher-sandbox/cluster-api-addon-provider-fleet/actions/runs/14619424871/job/41016176936

Run gh release create v0.8.0 --generate-notes --discussion-category "Announcements" metadata.yaml _out/addon-components.yaml
HTTP 404: Discussion could not be created. Make sure you passed a valid category name. (https://api.github.com/repos/rancher-sandbox/cluster-api-addon-provider-fleet/releases/214231238)
Error: Process completed with exit code 1.