[stdlib] Update MigrationSupport.swift

[benrimmington]

Deprecated APIs can be obsoleted in Swift 6 language mode.

• SE-0187 flatMap
• SE-0204 index(where:) and index(of:)
• SE-0214 DictionaryLiteral
• SE-0241 encodedOffset and init(encodedOffset:)

Playground APIs cannot be obsoleted.

error: unavailable property 'customPlaygroundQuickLook' was used to satisfy a requirement of protocol '_CustomPlaygroundQuickLookable'