[repo-assist] Forward NoWarn/TreatWarningsAsErrors/WarningsAsErrors to PackTask

[github-actions]

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Closes #3504

Root cause

Paket.Restore.targets invokes MSBuild's PackTask for dotnet pack but never forwards the NoWarn, TreatWarningsAsErrors, or WarningsAsErrors properties, even though NuGet's PackTask.cs has exposed these parameters since NuGet.Client 4.7 (NuGet/NuGet.Client#2013). Consequently, warnings such as NU5105 (SemVer 2.0 pre-release version string) could not be suppressed via NoWarn when packing through Paket.

Fix

Forward $(NoWarn), $(TreatWarningsAsErrors), and $(WarningsAsErrors) MSBuild properties to the PackTask invocation guarded by UseMSBuild16_10_Pack — the current, actively used pack path for modern MSBuild/SDK versions. The older legacy PackTask invocations (UseMSBuild16_0_Pack, UseMSBuild15_9_Pack, UseMSBuild15_8_Pack, UseNuGet4_Pack) predate this NuGet.Client support and are intentionally left unchanged, matching the issue reporter's own caution about not blindly passing these to versions that may not support them.

Trade-offs

• Only the newest pack path gets the fix; users on older MSBuild/NuGet.Client toolchains (rare today) won't get suppression support, but also won't risk passing unsupported task parameters.
• Minimal, surgical change to a single embedded MSBuild targets file; no F# code changes.

Test Status

• Validated the modified Paket.Restore.targets is well-formed XML (xml.dom.minidom parse succeeded).
• Could not run a full dotnet build/pack integration test in this sandbox because dotnet paket restore requires network access to NuGet feeds that isn't available here. No existing automated integration test scenario covers dotnet pack warning suppression, so none could be extended without adding new test infrastructure, which is out of scope for this minimal fix.
• Change is additive (new optional attributes on an existing task invocation) and does not alter existing behavior for projects that don't set NoWarn/TreatWarningsAsErrors/WarningsAsErrors.

Please review CI results after this PR is opened to confirm no regressions in the existing Windows/.NET and Linux build+test suites.

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