GH-47131: [C#] Fix day off by 1 in Date64Array

[jeremycostanzo]

Rationale for this change

Date64Array.Convert(DateTimeOffset) substracts one day on date times that are at 00:00 am and < 1970.
For example, 01/01/1969 00:00:00 gets converted to 12/31/1968 whereas the expected result is 01/01/1969.

What changes are included in this PR?

Fix to the described bug.

Are these changes tested?

Yes

Are there any user-facing changes?

No

This PR contains a "Critical Fix".

• GitHub Issue: [C#] Off by 1 in Date64Array #47131

[github-actions]

⚠️ GitHub issue #47131 has been automatically assigned in GitHub to PR creator.

[jeremycostanzo]

With the added test cases and without the fix,
Date64ArrayTest.AppendGivesUtcDate(dateTimeOffset: 1969-12-31T00:00:00.0000000+00:00) fails with

Assert.Equal() Failure: Values differ
Expected: 1969-12-31T00:00:00.0000000Z
Actual:   1969-12-30T00:00:00.0000000

Date64ArrayTest.AppendGivesUtcDate(dateTimeOffset: 0001-01-01T00:00:00.0000000+00:00) fails with

System.ArgumentOutOfRangeException: Valid values are between -62135596800000 and 253402300799999, inclusive. (Parameter 'milliseconds')