Remove always-on language feature flag: PreferStringGetPinnableReference

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 221 in ea778bb

LanguageFeature.PreferStringGetPinnableReference, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1612 in ea778bb

featurePreferStringGetPinnableReference,"prefer String.GetPinnableReference in fixed bindings"

Part of #20139