Update references from g_delivery to g_release_and_deploy

This MR updates all references to the g_delivery Slack channel to g_release_and_deploy to reflect the team's channel rename.

Related to https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/21527

Changes

Updated the following files:

• alertmanager/alertmanager.jsonnet - Updated comment for release management alerts routing
• docs/uncategorized/omnibus-package-updates.md - Updated Slack channel reference in troubleshooting section
• docs/registry/db-post-deployment-migrations.md - Updated Slack channel reference for contacting release managers