Quadruple limits for parallel and matrix permutations

What does this MR do and why?

Implements a customer suggestion from gitlab#336576 (Consider increasing limit of parallel keyword).

Initial version of this change was:

• add docs changes cc @marcel.amirault
• amend Danger-requested changes
• push schema/ci.json changes

The current version aims to additionally make the increase subscription-dependent, as discussed below.

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