Remove link to deprecated v1 plugins.

[Phil-Ah]

Proposed changes

Remove superfluous files and fragments. Change headline of plugin center to reflect removal. Add hint and link to scm-github for sources of v1 plugins. Resolves #97

Your checklist for this pull request

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Contributor:

• [x] PR is well described and the description can be used as a commit message on squash
• [x] Related issues linked to PR if existing and labels set
• [ ] New code is covered with unit tests
• [ ] New/updated ui components are tested inside the storybook (module ui-components only)
• [x] Changelog entry file created in gradle/changelog
• [ ] Feature has been tested with different permissions
• [ ] Documentation updated (only necessary for new features or changed behaviour)

Reviewer:

• [ ] The clean code principles are respected (CleanCode)
• [ ] All new code/logic is implemented on the right spot / "Should this be done here?"
• [ ] UI changes fits into the layout
• [ ] The UI views / components are responsive (mobile views)
• [ ] Correct translations are available

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• [ ] Branch is green/blue on Jenkins
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