Show mod version upgrade and changelog for Update

[dreamsyntax]

• Adds a scrollview to the update messagebox, and appends the changelog if available
• Adjust the mod update message to also include the current version and new version, to closer match HMM 7.
• This PR doesn't add markdown parsing. We can either manually parse using something like Markdig, or render it as a webview with an Avalonia.WebView package.

Before

After