Migrating `Common::CaseInsensitiveLess` to StringUtil.h will hopefully discourage rolling one's own solution in the future for case-insensitive associative containers when this (quite robust!) solution already exists.
`Common::CaseInsensitiveStringCompare::IsEqual` was removed in favor of using the `Common::CaseInsensitiveEquals` function.
The `a.size() != b.size()` condition in `Common::CaseInsensitiveEquals` can be removed, since `std::ranges::equal` already checks this condition (confirmed in libc++).
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