update startpage cta button

What does this MR do?

Update the startpage CTA from "Mitmachen" to "Jetzt mitmachen"

How confident are you it won't break things if deployed?

99,9% (0,1% for failing tests but I could not find any)

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Open startpage
2. See CTA

Screenshots (if applicable)

Before:

After:

Checklist

• added a test, or explain why one is not needed/possible... | I could not find any frontend tests and I probably should not just start them right away
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text

Update the startpage CTA from "Mitmachen" to "Jetzt mitmachen"