Feature gate `KeyedAccount` removal from builtin programs

[Lichtso]

Problem

KeyedAccount was replaced by BorrowedAccount in all built-in programs in v1.11. These refactorings should not change the behavior of the programs but because of the recent incident (#27740) we decided to feature gate them anyway, as we can't completely rule out changes such as reordering of error priorities or account aliasing constraints.

Summary of Changes

Takes all built-in program code as it was in c83c95b56b375afeea231b45cb4ced0cf07761e8, the parent commit of #23863, which marks the start of the refactorings, and has them switched to their new implementations in a single feature gate.

Feature Gate Issue: TODO