Remove minimum 4 digit requirement from Year.parse()

As per this question [1], the method Year.parse(CharSequence) is unclear in its behaviour for years before 1000.

When the method was written, the code for parsing was copied from other methods, such as LocalDate.parse(CharSequence). In those other methods, there is always a month, and often more fields, thus the year is forced to be four digits or more. But for Year.parse(CharSequence) this restriction is not as essential.

Option 1: Change the method spec to add a line something like "The year must have at least four digits, thus year '981' must be passed in as '0981'."

Option 2: Change the behaviour to allow any number of digits.

Note that option 2 would mean that Year.parse("999") would work, but YearMonth.parse("999-05") would not. But I still think this is the right choice. We don't have a Month.parse(CharSequence) method (which is an oversight), but if we did that method would not require the month to be zero prefixed.

[1] https://stackoverflow.com/questions/61907458/jsr310-year-parse-throws-datetimeparseexception-with-values-1000