fix: Revert "chore: upgrade to Gradle 8.0 (#89)" (#105)

[paullatzelsperger]

This reverts commit 86bc82f.

What this PR changes/adds

The previous reversion had changes in it, which we should not have reverted.
Thus, I downgraded manually to Gradle 7.6.

Why it does that

fix the build again

Further notes

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Linked Issue(s)

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Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• assigned appropriate label? (exclude from changelog with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and Etiquette for pull requests for details)