Remove always-on language feature flag: CSharpExtensionAttributeNotRequired

[T-Gro]

LanguageFeature.CSharpExtensionAttributeNotRequired has shipped since F# 8.0 and is enabled for every selectable language version (the minimum accepted --langversion is 8.0). Its feature flag can therefore never be turned off, so it is dead configuration. This issue removes the flag and collapses the guarded code to its enabled behaviour.

What to remove

• The CSharpExtensionAttributeNotRequired case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureCSharpExtensionAttributeNotRequired string in FSComp.txt
• Every SupportsFeature LanguageFeature.CSharpExtensionAttributeNotRequired / checkLanguageFeatureAndRecover … CSharpExtensionAttributeNotRequired guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

Consequence: --disableLanguageFeature:CSharpExtensionAttributeNotRequired is no longer a recognised feature name.

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 201 in ea778bb

LanguageFeature.CSharpExtensionAttributeNotRequired, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1595 in ea778bb

featureCSharpExtensionAttributeNotRequired,"Allow implicit Extension attribute on declaring types, modules"

Part of #20139