perf: drop redundant char* fixed in MessagePackWriter.Write(string) - #2258

On SPAN_BUILTIN targets (netstandard2.1/net8.0/net9.0), use the span-based Encoding.GetBytes(ReadOnlySpan<char>, Span<byte>) overload so the fixed (char* pValue = value) pin can go away. The Encoding API pins the source itself for the duration of the encode, which is shorter than holding the pin across WriteString_PostEncoding.

Legacy targets (netstandard2.0/net472) keep the original fixed (char* …) path under #else.

The fixed (byte* pBuffer = &buffer) stays for now — WriteString_PostEncoding still takes byte* and uses Buffer.MemoryCopy for the overlapping prefix shift. Dropping that pin too would mean rewriting WriteString_PostEncoding to ref byte + MemoryMarshal.CreateSpan(...).CopyTo(...), but MemoryMarshal.CreateSpan is netstandard2.1+ only (not in the System.Memory backport for netstandard2.0/net472), so that's a bigger refactor for another PR.

Test plan

• Builds clean on all 5 TFMs (netstandard2.0;netstandard2.1;net8.0;net9.0;net472)
• MessagePack.Tests — 999/999 pass on net9.0, 968/968 on net472