Need to add note about `System.Drawing.Common` being deprecated in Linux in .NET 7

[udlose]

According to @svick noted here (dotnet/runtime#80148), the support for System.Drawing.Common has now been completely deprecated in .NET 7; thus, the System.Drawing.EnableUnixSupport setting in runtimeconfig.template.json is no longer valid.

The list of Breaking changes in .NET 7 does not include any information about the deprecation of Unix support for System.Drawing.Common or the deprecation of the System.Drawing.EnableUnixSupport config setting. This info should be added to the list of Breaking Changes so that developers are aware of this change.

---

Document Details

⚠ Do not edit this section. It is required for learn.microsoft.com ➟ GitHub issue linking.

• ID: fda38618-d1fc-5b45-9a09-1a02643d99ae
• Version Independent ID: e59e516b-dd93-937d-f9a1-bce4b5eb3aa9
• Content: Breaking changes in .NET 7 - .NET
• Content Source: docs/core/compatibility/7.0.md
• Product: dotnet-fundamentals
• GitHub Login: @gewarren
• Microsoft Alias: gewarren

[gewarren]

@udlose This was technically a breaking change in .NET 6, not .NET 7, and as such is listed here: https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/6.0/system-drawing-common-windows-only. This article also contains a note that the switch to re-enable support was removed in .NET 7 (and I don't believe the removal of the switch is considered an additional breaking change).

[gewarren]

Related to #33653.