Remove always-on language feature flag: ReallyLongLists

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 190 in ea778bb

LanguageFeature.ReallyLongLists, languageVersion70

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1587 in ea778bb

featureReallyLongList,"list literals of any size"

Part of #20139