Test against .NET 11.0.1xx only and disable SDK Next integration tests by default since we're only targeting 11.0.1xx now.

[nkolev92]

Bug

Fixes:

Progress: https://github.com/NuGet/Client.Engineering/issues/3874

Description

Test/CI configuration change only (no product code) in preparation for the 7.9 branch. Moves Dotnet.Integration.Test onto the .NET 11.0.1xx SDK and turns the SDK Next test legs off by default so they don't gate PRs while SDK Next does not exist. Nothing is deleted — SDK Next remains easy to re-enable.

• eng/pipelines/pr.yml — Added a new RunSdkNextTests pipeline parameter (default false) and gated the three Dotnet.Integration.Test ... (SDK Next) stages (Windows / Linux / MacOS) on it. The stages stay in the file and can be re-enabled by checking the parameter when flipping the default.
• test/NuGet.Core.FuncTests/Dotnet.Integration.Test/Dotnet.Integration.Test.csproj — Added a TestSDKNext property (default false) that controls whether the sdkNext target framework is built, plus an IsEscrowMode guard.
• build/DotNetSdkTestVersions.txt — Install only the .NET 11.0.1xx daily channel (removed 10.0.4xx).
• test/NuGet.Core.FuncTests/Dotnet.Integration.Test/TestConstants.cs — Bumped the test ProjectTargetFramework from net10.0 to net11.0.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.