AdvSimd intrinsic obsoletion

[RealityProgrammer]

As stated in this issue, AdvSimd implementation for pack/unpack/etc... will be obsoleted due to these following reasons:

• Lack of testing environment: ARM Neon is (as far as I know) majority used for mobile hardwares, which cannot be test.
• Source error: At the time of writing, AdvSimd is having some issue with incorrect constant range. Consider this following examples:

/// <summary>
/// uint64x2_t vshrq_n_u64 (uint64x2_t a, const int n)
///   A32: VSHR.U64 Qd, Qm, #n
///   A64: USHR Vd.2D, Vn.2D, #n
/// </summary>
public static Vector128<ulong> ShiftRightLogical(Vector128<ulong> value, [ConstantExpected(Min = 1, Max = (byte)(16))] byte count) { throw new PlatformNotSupportedException(); }

has incorrect range, which should be between 1 to 64 according to the official documentation of ARM.

These AdvSimd methods will not be deleted, but instead, they will be commented out, serve as a foundation once we decided to tackle AdvSimd.

[oliverbooth]

Switched base to release/4.0.0

[oliverbooth]

Ignore failure for .NET job on .NET Core 3.1. netstandard2.0 target is removed in 4.0.0. Also ignore the Unity test runner. As long as it builds on .NET 6+ we are good. However, source validation is failing