New Script: Proxmox Mail Gateway Post Installer

[thost96]

✍️ Description

Created a Post Installer Script for Proxmox Mail Gateway to fix APT Sources, Remove Enterprise Repo, Add No-Subscription Repo, Add Test Repo, Disable No-Subscription Nag on PMG, Update System and Reboot if neccessary. The Script works like the PBS Post Installer or the PVE Post Installer.

✅ Prerequisites

The following steps must be completed for the pull request to be considered:

• Self-review performed (I have reviewed my code to ensure it follows established patterns and conventions.)
• Testing performed (I have thoroughly tested my changes and verified expected functionality.)

🛠️ Type of Change

Please check the relevant options:

• [] Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to change unexpectedly)
• New script (a fully functional and thoroughly tested script or set of scripts)

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📋 Additional Information (optional)

Provide any extra context or screenshots about the feature or fix here.

As this is my frist Contribution to this Project, i hope i haven't missed anything.