Fix handling of getter-only properties on [Key] types

[AArnott]

Also fix several issues found during migration.

• Improve NBMsgPack001 when there is no syntax for the member
• Fix ignoring of getter-only properties in [Key] mode
• Fix trivia preservation around changed/removed attributes
• Fix migration of nested converters

[codecov]

Codecov Report

Attention: Patch coverage is 58.17308% with 87 lines in your changes missing coverage. Please review.

Project coverage is 78.87%. Comparing base (d0074f2) to head (12ac63b).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...sagePack.Analyzers.Tests/MigrationAnalyzerTests.cs	17.28%	67 Missing ⚠️
...k.MessagePack.Analyzers/KeyAttributeUseAnalyzer.cs	70.83%	5 Missing and 2 partials ⚠️
...Nerdbank.MessagePack.Tests/ObjectsAsArraysTests.cs	77.27%	5 Missing ⚠️
...essagePack.Analyzers.CodeFixes/MigrationCodeFix.cs	75.00%	2 Missing and 1 partial ⚠️
...Nerdbank.MessagePack.Analyzers/ReferenceSymbols.cs	62.50%	2 Missing and 1 partial ⚠️
...st/Nerdbank.MessagePack.Tests/ObjectsAsMapTests.cs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
- Coverage   78.98%   78.87%   -0.12%     
==========================================
  Files         130      130              
  Lines        8129     8223      +94     
  Branches     1123     1127       +4     
==========================================
+ Hits         6421     6486      +65     
- Misses       1378     1405      +27     
- Partials      330      332       +2     

Flag	Coverage Δ
Linux	78.87% <58.17%> (-0.12%)	⬇️

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