Reformat the attacks table, fix link to reqs.
Reformat the attacks table to make it easier to maintain: - Remove unnecessary closing `tr` and `td` tags. - Merge short columns onto one line. - Separate each row with a blank line. - Break lines at natural boundaries. Fix a stale link to build-requirements.md, which (a) was an absolute link instead of relative, (b) pointed to a file that has since been deleted, and (c) didn't work on slsa.dev since the jekyll-relative-links plugin only works on Markdown links, not HTML links.