Drop max-parallel from GitHub build

[Tibor17]

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[Tibor17]

@slawekjaranowski
Your commit is higly needed.
Can we proceed with this PR? Do you want to add something into it?

The reason why I used the latest JDK 18 for ff was the fact that new issues (similar to a problem with SecurityManager) can be discovered with latest JDK version. Therefore I used ff-jdk: '18-ea'.

[slawekjaranowski]

Finished - can be merged.