Add PPA for dotnet backport during image building

[yhaliaw]

Applicable spec:

Overview

Add the PPA for dotnet backports.

Rationale

Needed to install additional dotnet version for different ubuntu base.
E.g., jammy with dotnet 10, resolute with dotnet 8.

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, senior-review-required, documentation)
• The docs/changelog.md is updated with user-relevant changes in the format of keep a changelog v1.1.0
• The application version is incremented in app/pyproject.toml

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• canonical-bos01.cdn.snapcraftcontent.com
  • Triggering command: /usr/lib/snapd/snapd /usr/lib/snapd/snapd (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)