PAINTROID-786 Update AGP to 8.7.3

[foenabua]

https://catrobat.atlassian.net/jira/software/c/projects/PAINTROID/boards/130?selectedIssue=PAINTROID-786

Update Gradle Plugin from 7.2.1 to 8.9.1.
New gradle plugin version needs JAVA 17 and a new Docker image for Jenkins was provided by devops team for that purpose.
Please review the contributing guidelines and wiki pages of this repository.

• Include the name of the Jira ticket in the PR’s title
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• Include reasonable and readable tests verifying the added or changed behavior
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