[FIX] orm: performance of regex to check search `order` spec

This regex is used for a quick sanity check of
the order_spec in `search(order=<order_spec>)`.
Because it was build on the repetition of a
group ending with a series of optional patterns,
it could cause expensive backtracking when the
order spec did not actually match the regex
(the regex engine was trying all possible ways
to split the groups)

Forcing the repeating group to either end
with a comma or the end of the string prevents
prohibitive backtracking, while being even
more restrictive with regard to the syntax of
the order spec.

Closes #7755

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