Address Copilot review: pin Net80 refs and fix stale comment

- Skips_StartsWith_CharVariable depends on string.StartsWith(char),
  which is .NET Core 2.0+. Pin ReferenceAssemblies.Net.Net80 to make
  the contract explicit and match the precedent set by the 2xxx tests.
- The "string first parameter" comment was stale after the helper
  was renamed to ValueParameterIsString (which skips the receiver
  for non-reduced extension calls); reword to "string value parameter".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: paulirwin

src/Lucene.Net.CodeAnalysis.Dev/LuceneDev6xxx/LuceneDev6001_6002_StringComparisonAnalyzer.cs

@@ -175,7 +175,7 @@ static bool ValueParameterIsString(IMethodSymbol? m)
             // Handle ambiguous candidates
             if (candidateSymbols.Length > 0)
             {
-                // Check if any candidate is from String or J2N types and takes a string first parameter
+                // Check if any candidate is from String or J2N types and takes a string value parameter
                 var relevantCandidates = candidateSymbols
                     .Where(c => ContainingTypeIsStringOrJ2N(c.ContainingType) && ValueParameterIsString(c))
                     .ToImmutableArray();

tests/Lucene.Net.CodeAnalysis.Dev.Tests/LuceneDev6xxx/TestLuceneDev6001_6002_StringComparisonAnalyzer.cs

@@ -146,6 +146,7 @@ public void M()
             var test = new InjectableCSharpAnalyzerTest(() => new LuceneDev6001_6002_StringComparisonAnalyzer())
             {
                 TestCode = testCode,
+                ReferenceAssemblies = ReferenceAssemblies.Net.Net80, // string.StartsWith(char) requires .NET Core 2.0+
                 ExpectedDiagnostics = { } // StartsWith(char) has no StringComparison overload; no diagnostic
             };