Restrict to have only one top-level describe

What does this MR do and why?

Restrict to have only one top-level describe.

Adds an entry to no-restricted-syntax

A big disadvantage of merging this is that entire files would ignore the whole no-restricted-syntax even if they only offend the top-level describe rule.

References

• Idea: https://stackoverflow.com/a/71938202

Screenshots or screen recordings

NA

MR acceptance checklist

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Related to #118585