Fix CI failures on to_timestamp() calls (#7941)

* Change input for `to_timestamp` function

* docs

* fix examples

* output `to_timestamp` signature as ns

* Fix CI `to_timestamp()` failed

* Update datafusion/expr/src/built_in_function.rs

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

* fix typo

* fix

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

datafusion/expr/src/built_in_function.rs

@@ -755,7 +755,10 @@ impl BuiltinScalarFunction {
                     return plan_err!("The to_hex function can only accept integers.");
                 }
             }),
-            BuiltinScalarFunction::ToTimestamp => Ok(Timestamp(Nanosecond, None)),
+            BuiltinScalarFunction::ToTimestamp => Ok(match &input_expr_types[0] {
+                Int64 => Timestamp(Second, None),
+                _ => Timestamp(Nanosecond, None),
+            }),
             BuiltinScalarFunction::ToTimestampMillis => Ok(Timestamp(Millisecond, None)),
             BuiltinScalarFunction::ToTimestampMicros => Ok(Timestamp(Microsecond, None)),
             BuiltinScalarFunction::ToTimestampNanos => Ok(Timestamp(Nanosecond, None)),