Update target framework visibility conditions and remove VS-specific error message

[Copilot]

Description:

Two items in this PR.
Currently customers in VS 17.14 will see net10 in the project system dropdown even though they can't actually target it without a warning.
After team discussion, we decided to simplify error handling by removing the VS-specific error message (NETSDK1209) that was too difficult to maintain.

Customer impact

Customers will no longer see net10.0 when using VS 17.14 so they can't accidentally retarget and then get warnings that it's not supported.
All customers will now receive a consistent error message when targeting unsupported framework versions, regardless of whether they're using Visual Studio or not.

Error Message
The current .NET SDK does not support targeting {0} {1}. Either target {0} {2} or lower, or use a version of the .NET SDK that supports {0} {1}. Download the .NET SDK from https://aka.ms/dotnet/download

Regression

No

Risk

Low

Align target framework visibility with VS/MSBuild version support to prevent users from seeing unsupported frameworks in older VS versions, and simplify error handling by removing VS-version-specific logic.

Changes

• net9.0: Removed MSBuild 17.12.0 version condition (now unconditionally visible)
• net10.0: Added MSBuild 18.0.0+ version condition
• MinimumVisualStudioVersionForUnsupportedTargetFrameworkVersion: Removed (no longer needed)
• DisplayVSMessage logic: Removed from Microsoft.NET.TargetFrameworkInference.targets
• UnsupportedVisualStudioVersion resource (NETSDK1209): Removed
• Error code 1209: Added to deleted codes list in GivenThatWeHaveErrorCodes.cs
• Added comment explaining conditional inclusion rationale and VS/MSBuild version relationship

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[marcpopMSFT]

@baronfel I'm trying to decide what to do with UnsupportedVisualStudioVersion as if I fix this, it'll tell customers to install 19.0 which is wrong for targeting 11 but if I set it to anything else, I have to remember to update it later. I wonder if the other error message is sufficient and we should just delete this one as it's too much of a hassle to upkeep.

[baronfel]

@marcpopMSFT maybe we should reconsider where this message lives - perhaps the project system, especially with that new targeting dialog that VS added in dev18, would be a better solution for the problem we're trying to solve here?

[phenning]

@baronfel , @JakeRadMSFT lets start a mail thread and discuss this offline.

[marcpopMSFT]

@copilot after discussion we decided that we no longer want to have a specific VS warning message.
Instead, we want the following.

• Remove MinimumVisualStudioVersionForUnsupportedTargetFrameworkVersion from this file
• Remove DisplayVSMessage from Microsoft.NET.TaretFrameworkInference.targets
• Remove the UnsupportedVisualStudioVersion resource
• Find and update the test that tracks warning codes in GivenThatWeHaveErrorCodes.cs to mark 1209 as an expected missing code.

[Copilot]

@copilot after discussion we decided that we no longer want to have a specific VS warning message.
Instead, we want the following.

• Remove MinimumVisualStudioVersionForUnsupportedTargetFrameworkVe...

Done in commit 2c2779f. Removed:

• MinimumVisualStudioVersionForUnsupportedTargetFrameworkVersion property
• DisplayVSMessage logic from Microsoft.NET.TargetFrameworkInference.targets
• UnsupportedVisualStudioVersion resource (NETSDK1209)
• Added 1209 to deleted error codes list in GivenThatWeHaveErrorCodes.cs

All tests passing.