Add workflow to update Gradle wrapper automatically, update to 6.7

[EwoutH]

• Adds workflow that checks every morning if a new Gradle version is available, and will open a PR if so
• Updates the Gradle Wrapper to that version using that workflow

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

:white_check_mark: EwoutH
:x: EwoutH GradleUp Action

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EwoutH GradleUp Action seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[EwoutH]

The CLAassistant could be a problem here, since currently these commits will not be signed. Adding *GradleUp Action to the allow list could solve this problem, but I can see why this is not preferable.

Let me check if there are bots that use signed commits, maybe Dependabot will work.

[EwoutH]

Closing this for now, will add a Dependabot config when Gradle with Kotlin is supported https://github.com/dependabot/dependabot-core/issues/2238.

For now succeeded by #41.