Remove always-on language feature flag: InitPropertiesSupport

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 193 in ea778bb

LanguageFeature.InitPropertiesSupport, languageVersion70

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1592 in ea778bb

featureInitProperties,"support for consuming init properties"

Part of #20139