Add conversion to DateOnly from DateTime and DateTimeOffset

[mkorsukov]

Hi .NET team and community!

I found the DateOnly type is so useful in daily use cases, so I think about an addition to DateTime and DateTimeOffset types in the form of ToDateOnly() instance method.

Here is code snippets:

DateTime.cs

public DateOnly ToDateOnly() =>
    DateOnly.FromDateTime(this);

DateTimeOffset.cs

public DateOnly ToDateOnly() =>
    DateOnly.FromDateTime(_dateTime);

Can't create a PR, so unit tests included.

DateTimeTests.cs

[Fact]
public static void ToDateOnly()
{
    DateTime dateTime = new DateTime(2024, 06, 13, 11, 55, 45, 0, DateTimeKind.Utc);
    Assert.Equal(new DateOnly(2024, 06, 13), dateTime.ToDateOnly());
}

DateTimeOffsetTests.cs

[Fact]
public static void ToDateOnly()
{
    DateTimeOffset dateTimeOffset = new DateTimeOffset(new DateTime(2024, 06, 13, 11, 55, 45, 0, DateTimeKind.Utc));
    Assert.Equal(new DateOnly(2024, 06, 13), dateTimeOffset.ToDateOnly());
}

[colejohnson66]

API requests go in the issues section

[huoyaoyuan]

Duplicate of #58734