Fix regex in multiline links vale test

What does this MR do?

As reported in the related issue, there were some false positives from this test, mostly due to code blocks that use square brackets for things that aren't markdown links. Telling the regex to avoid both closing and opening square brackets seems to fix the test, and all the false positives cleared up.

• Before: https://regex101.com/r/A0r5Ld/2
• After: https://regex101.com/r/A0r5Ld/3

Related issues

Related to #370065 (closed)

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