Fix ArXiVIdentifier -- dot is required and not arbitrary character

[koppor]

In #8129 @colinhex found out that we have a parsing error in org.jabref.model.entry.identifier.ArXivIdentifier. ArXiV requires a . (dot) and not an arbitrary character. See https://arxiv.org/help/arxiv_identifier.

This PR fixes it.

Moreover, I identified that Eprint is a semantic duplicate of ArXivIdentifier. The former was implemented by @stefan-kolb on 2017-03-09, the latter by @tobiasdiez on 2017-01-15. I removed Eprint and integrated the tests in ArXivIdentifierTest. I also fixed some minor issues in ArXivIdentifier.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[tobiasdiez]

Ohh, good that you spotted the duplication in the Eprint and Arxiv identifier.

[Siedlerchr]

A fetcher test is failing:

  org.opentest4j.AssertionFailedError: Expected org.jabref.logic.importer.FetcherException to be thrown, but nothing was thrown.
      at app//org.junit.jupiter.api.AssertThrows.assertThrows(AssertThrows.java:71)
      at app//org.junit.jupiter.api.AssertThrows.assertThrows(AssertThrows.java:37)
      at app//org.junit.jupiter.api.Assertions.assertThrows(Assertions.java:3082)
      at app//org.jabref.logic.importer.fetcher.ArXivTest.searchWithMalformedIdThrowsException(ArXivTest.java:226)

[koppor]

Tests failing, because IEEE is down:

Example link: https://ieeexplore.ieee.org/document/4279612

[koppor]

checkstyle should be fixed by #8154