Updated german translation for 1.21.0

[tweimer]

Check if this PR fulfills these requirements:

• My changes respect the Kodi add-on development rules
• I have read the CONTRIBUTING document.
• I made sure there wasn't another Pull Request opened for the same update/change
• I have successfully tested my changes locally

Types of changes

• New feature (non-breaking change which adds functionality)
• Feature change (non-breaking change which changes behaviour of an existing functionality)
• Improvement (non-breaking change which improves functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (non-breaking performance or readability improvements)

Description

Unfortunately the release of 1.21.0 is already done.

See #306

In case of Feature change / Breaking change:

Describe the current behavior

Describe the new behavior

Screenshots (if appropriate):