This PR fixes two fetchers (refs #6369):

• APS fetcher: \textbackslash is added server side --> removed at cleanup-phase
• ScienceDirect: location of PDF link changed --> rewrote code to extract the PDF url - fixes ScienceDirect Fulltext download does not work #5860

• Change in CHANGELOG.md described (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 created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.