fix(date): fix birthdate

[ST-DDT]

Fixes #2783

• 2783

There are actually two errors:

• One where it is possible to generate the exact same date as the refDate (aka 2020-03-31 and 2000-03-31 for age 20)
• The other lies in the test, where the age is calculated wrongly if the refDate is within a leap year and after Feb 29

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.95%. Comparing base (0fe5af8) to head (600173d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## next #2829 +/- ## ========================================== - Coverage 99.95% 99.95% -0.01% ========================================== Files 2964 2964 Lines 212541 212544 +3 Branches 600 597 -3 ========================================== + Hits 212455 212457 +2 - Misses 86 87 +1 ``` | [Files](https://app.codecov.io/gh/faker-js/faker/pull/2829?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=faker-js) | Coverage Δ | | |---|---|---| | [src/modules/date/index.ts](https://app.codecov.io/gh/faker-js/faker/pull/2829?src=pr&el=tree&filepath=src%2Fmodules%2Fdate%2Findex.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=faker-js#diff-c3JjL21vZHVsZXMvZGF0ZS9pbmRleC50cw==) | `100.00% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/faker-js/faker/pull/2829/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=faker-js)