Fix timestamp_add_interval_months to pass on any date #6814

Describe the bug

Ref #3321

timestamp_add_interval_months fails since month is 7, result is 0.

https://github.com/apache/arrow-datafusion/blob/84832ac3f13af78336f3fa291e88e05f3909faac/datafusion/core/tests/sql/timestamp.rs#L583-L586

To Reproduce

Test this in July.

Expected behavior

Pass the test whenever the date we run it.

Additional context

No response