Fix ECMA standard warnings

[BillWagner]

Fix the warnings that crept in from the branch updates following the July ECMA meeting.

---

Internal previews

File	Preview link
docs/csharp/language-reference/builtin-types/default-values.md	docs/csharp/language-reference/builtin-types/default-values
docs/csharp/language-reference/builtin-types/record.md	docs/csharp/language-reference/builtin-types/record
docs/csharp/language-reference/builtin-types/ref-struct.md	docs/csharp/language-reference/builtin-types/ref-struct
docs/csharp/language-reference/builtin-types/struct.md	docs/csharp/language-reference/builtin-types/struct
docs/csharp/language-reference/compiler-messages/record-declaration-errors.md	docs/csharp/language-reference/compiler-messages/record-declaration-errors
docs/csharp/language-reference/compiler-messages/ref-modifiers-errors.md	docs/csharp/language-reference/compiler-messages/ref-modifiers-errors
docs/csharp/language-reference/operators/equality-operators.md	docs/csharp/language-reference/operators/equality-operators
docs/csharp/language-reference/operators/with-expression.md	docs/csharp/language-reference/operators/with-expression
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error
docs/csharp/programming-guide/generics/constraints-on-type-parameters.md	docs/csharp/programming-guide/generics/constraints-on-type-parameters

[Copilot]

Pull request overview

This PR updates the C# compiler error reference page to remove frontmatter content that triggers ECMA/doc build warnings introduced after recent branch updates.

Changes:

• Removes a block of commented “C# 8.0 diagnostics” headings from the f1_keywords YAML frontmatter.
• Keeps the remaining keyword grouping comments for newer compiler diagnostics.