The ioutil functions are deprecated since Go 1.17 and only wrap another
library function. Thus directly call the underlying function.
This commit only mechanically replaces the function calls.
Sometimes restic gets bogus timestamps which cannot be converted to
JSON, because the stdlib JSON encoder returns an error if the year is
not within [0, 9999]. We now make sure that we at least record _some_
timestamp and cap the year either to 0000 or 9999. Before, restic would
refuse to save the file at all, so this improves the status quo.
This fixes#2174 and #1173
