ci: Fix paths filter

[n0samu]

• The | glob operator does not actually work. As a workaround, switch to a fork of the paths-filter action that supports multiple exclude rules. (dorny/paths-filter does allow you to specify multiple exclude rules, but it matches any of them instead of all of them, which is useless for us.)
• Our exclude rule for markdown files was wrong - it only matched files in the top-level directory. Correct the rule to also match markdown files in subdirectories.

For more context, see:

• Excluding file via negation is not working as expected dorny/paths-filter#135
• dev: run workflows only if certain file paths change #5103
• ci: Fix paths filter #5212
• dev: have required builds 'run' #5332