Preserve literal-format Forms Authentication password hashes

Opt-in .NET 5+ port of resolved FormsAuthentication.HashPasswordForStoringInConfigFile calls with literal SHA1, MD5, SHA256, SHA384 or SHA512 formats. Preserve UTF8 input, uppercase hexadecimal output, null-password errors and single evaluation. This preserves legacy stored hashes; it does not strengthen password storage. Platform cryptographic providers and policy may differ. Named arguments, expression trees, conditional files and commented discarded syntax remain for review. Unused top-level legacy imports are removed conservatively.