Update links to the boulder repo to link to the main branch

[magnuswatn]

Stumbled upon some links to the boulder repo that linked to an outdated version, becasue it linked to the master branch, and not the main branch. Thought that I might as well just update all the links to the boulder repo at once.

I did not bump lastmod on purpose, as I also changed the translated versions, and did not want to to trigger unnecessary "this is outdated" messages, or hide the need for real updates. Just let me know if I should bump it.

[tdelmas]

I think it's better to wait for https://github.com/letsencrypt/website/pull/1433 to be merged, to avoid complicated conflicts. (Ping @ashmoremsp)

For lastmod, both solutions seams good for me.