Remove always-on language feature flag: ExtendedStringInterpolation

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 212 in ea778bb

LanguageFeature.ExtendedStringInterpolation, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1605 in ea778bb

featureExtendedStringInterpolation,"Extended string interpolation similar to C# raw string literals."

Part of #20139