Remove always-on language feature flag: RequiredPropertiesSupport

[T-Gro]

LanguageFeature.RequiredPropertiesSupport has shipped since F# 7.0 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 RequiredPropertiesSupport case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureRequiredProperties string in FSComp.txt
• Every SupportsFeature LanguageFeature.RequiredPropertiesSupport / checkLanguageFeatureAndRecover … RequiredPropertiesSupport guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 192 in ea778bb

LanguageFeature.RequiredPropertiesSupport, languageVersion70

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1591 in ea778bb

featureRequiredProperties,"support for required properties"

Part of #20139