Make existing PBKDF2 SHA-1 and 1000 iterations explicit

Replace attributed Rfc2898DeriveBytes(string, byte[]) construction with the explicit legacy defaults 1000 and HashAlgorithmName.SHA1. Preserve derived bytes, stateful GetBytes/Reset, salt and object initialization. Require a string-typed password expression to avoid new overload ambiguity. Named arguments, generated salts, other signatures, unresolved calls and conditional snapshots stay unchanged. Opt-in compatibility migration; does not strengthen passwords or select new hash/iteration policies. Lambda/query contexts remain unchanged.