Set correct UUID variant bits in uuidBytes array

Updated manipulation of the 9th byte in uuidBytes to mask with 0x3F and set the two highest bits to '10', ensuring compliance with the UUID variant specification.

The #else branch (all sub-net8 TFMs) built uuidBytes in RFC 9562
big-endian order but constructed the Guid with new Guid(byte[]), which
reads the first three fields as little-endian. This byte-swapped the
first three groups in canonical form: the version nibble landed at the
wrong position and the timestamp bytes were scrambled, producing
non-canonical v7 GUIDs. Reverse the first three fields before
construction so the result matches the bigEndian path used on
NET8_0_OR_GREATER.
Also expand Guid test coverage:
- assert CreateVersion7 sets the version (7) and RFC 9562 variant bits
- assert CreateVersion7(DateTimeOffset) encodes the timestamp exactly
- cover the 2-arg TryParse(ReadOnlySpan<char>, out) overload (valid + invalid)
- add a TryParseExact failure case

…#553)

* Add AsyncMethodBuilderAttribute  and CompilerLoweringPreserveAttribute

* .