fix(mergify): remove deprecated speculative_checks option

[gustavovalverde]

Motivation

While using Mergify's configuration checker, this issue was not raised, but it's now showing in some PR's summaries, as in https://github.com/ZcashFoundation/zebra/pull/9026/checks?check_run_id=33066692861

Specifications & References

• https://docs.mergify.com/merge-queue/parallel-checks/

Solution

• Use the new root level option max_parallel_checks

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.