datetime: simplify boundary checks

We used to use very ugly and tricky approach to check that passed years,
months and days were not exceeding supported range of values. Now we have
introduced to `c-dt` library the new function `dt_from_ymd_checked` for
that purpose (i.e. check that values are valid, and construct dt from
them). So rewrite/simplify Lua code to use that entry as
`tnt_dt_from_ymd_checked`.

Part of #6731

NO_DOC=refactoring
NO_CHANGELOG=refactoring