Remove always-on language feature flag: EscapeDotnetFormattableStrings

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 204 in ea778bb

LanguageFeature.EscapeDotnetFormattableStrings, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1727 in ea778bb

featureEscapeBracesInFormattableString,"Escapes curly braces before calling FormattableStringFactory.Create when interpolated string literal is typed as FormattableString"

Part of #20139