build: TFM-warning + slnx + PublicAPI hygiene

[Chris-Wolfgang]

Summary

Three build-hygiene fixes from the code review.

(a) netcoreapp3.1 TFM-support warnings

Both test projects list netcoreapp3.1 in <TargetFrameworks> but their SuppressTfmSupportBuildWarnings PropertyGroups only covered net4x and net5/6/7. Extended the net5/6/7 PropertyGroup Condition in each to also match netcoreapp3.1. The 6 TFM-support warnings (3 TFMs × 2 test projects) are now gone.

(b) Stale .slnx references

Removed solution-file references to files that no longer exist, and fixed one wrong extension:

• removed REPO-INSTRUCTIONS.md
• removed .github/version-picker-template.html
• removed scripts/Setup-BranchRuleset.ps1
• removed scripts/Setup-GitHubPages.ps1
• .github/workflows/codeql.yml → .github/workflows/codeql.yaml (the real file)

Each removed path was confirmed missing; the corrected codeql.yaml confirmed present.

(c) PublicAPI.Unshipped sort

PublicAPI.Unshipped.txt in both src projects was not canonically sorted. Re-sorted each ordinally (matching PublicAPI.Shipped.txt's convention) with the #nullable enable header preserved as the first line.

Verification

dotnet build ETL-Test-Kit.slnx -c Release → 0 Warning(s), 0 Error(s) (TFM warnings gone, no RS0016/RS0017/RS0024/RS0025).

Stack

Based on fix/changelog (PR #180).

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