Limit certificates in rejected store to a constant value (currently 5)

[mrsuciu]

Proposed changes

The Rejected store currently keeps all the rejected certificates. This PR proposes that the Rejected store should keep only a limited value of the 5 last rejected certificates. It also eliminates the redundant check for existing certificates in the X509CertificateStore and DirectoryCertificateStore.

Related Issues

Types of changes

What types of changes does your code introduce? Put an x in the boxes that apply. You can also fill these out after creating the PR.

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] Enhancement (non-breaking change which adds functionality)
• [x] Test enhancement (non-breaking change to increase test coverage)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] I have read the CONTRIBUTING doc.
• [x] I have signed the CLA.
• [x] I ran tests locally with my changes, all passed.
• [ ] I fixed all failing tests in the CI pipelines.
• [ ] I fixed all introduced issues with CodeQL and LGTM.
• [ ] I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• [ ] I have added necessary documentation (if appropriate).
• [ ] Any dependent changes have been merged and published in downstream modules.

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.31579% with 9 lines in your changes missing coverage. Please review.

Project coverage is 54.53%. Comparing base (2aec785) to head (c967726). Report is 27 commits behind head on master.

Files with missing lines	Patch %	Lines
...Core/Security/Certificates/CertificateValidator.cs	72.41%	5 Missing and 3 partials :warning:
...cates/X509CertificateStore/X509CertificateStore.cs	87.50%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2693 +/- ## ========================================== - Coverage 54.66% 54.53% -0.13% ========================================== Files 347 347 Lines 65578 65602 +24 Branches 13442 13449 +7 ========================================== - Hits 35846 35777 -69 - Misses 25819 25898 +79 - Partials 3913 3927 +14 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[mregen]

I suggest to use #2720 instead, less complicated...