Upgrade to .Net Standard 2.1

[luispfgarces]

Description

Upgrades the packages under .Net Standard 2.0 to .Net Standard 2.1. Also upgrades some of the packages to newer versions and fixes an issue on failing tests (dependent on the machine culture).

Change checklist

• [x] Code follows the code rules guidelines of this project
• [x] Commit messages follow the commit rules of this project
• [x] I have self-reviewed my changes before submitting this pull request
• [x] I have covered new/changed code with new tests and/or adjusted existent ones -> no new code
• [x] I have made changes necessary to update the documentation accordingly -> no new code

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