[Repo Assist] fix: dispose SHA1 instance in Caching.hashString; use SHA1.HashData on .NET 5+

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Summary

Fixes a resource leak in Caching.fs — SHA1.Create() returned a HashAlgorithm (IDisposable) that was never disposed.

Before

let hashBytes = SHA1.Create().ComputeHash(plainTextBytes)
//              ^^^^^^^^^^^^ IDisposable never disposed

After

let hashBytes =
#if NET5_0_OR_GREATER
    SHA1.HashData(plainTextBytes)     // static, no instance, no disposal needed
#else
    use sha1 = SHA1.Create()          // properly disposed via 'use'
    sha1.ComputeHash(plainTextBytes)
#endif

Details

• netstandard2.0: Wraps the SHA1 instance in use for deterministic disposal — this is the HashAlgorithm.Dispose() path that frees any unmanaged resources.
• net8.0 (NET5_0_OR_GREATER): Uses SHA1.HashData(byte[]), the static API introduced in .NET 5 that avoids creating a disposable object entirely. This is the idiomatic modern approach.

hashString is called only at design-time (to derive a cache file name from a URL), so the performance impact is negligible — the value here is correctness and clean resource handling.

Test Status

• ✅ dotnet build src/FSharp.Data.Runtime.Utilities/ — 0 errors
• ✅ dotnet test tests/FSharp.Data.Core.Tests/ — 2920 passed, 0 failed
• ✅ Fantomas format check passed

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