Add RFC9116 security.txt check

[mxsasha]

• [x] ~~Currently fails as STATUS_FAIL, maybe this should be a lighter one like notice or info?~~ Explore using INFO for recommendations, WARN for others.
• [ ] The sectxt library is not yet publicly available so this will not build in CI and can not be deployed.
• [ ] Content TBD in https://github.com/internetstandards/Internet.nl_content/pull/23
• [ ] The text in the tech table is directly from the library and can use a bit of style improvement (PR created)
• [x] Would be nice to have some domains that have broken security.txt files. If we don't know of any, I can just make one.
• [x] Some of the naming in the code feels clunky and might need another look.
• ~~Look into making tech table translatable?~~
• [ ] ~~We currently do not have a consistency check between different hosts (including v4/v6)~~ -> out of scope for now, new issue
• [x] Raise file size limit to 100KB
• [x] Make sure we verify content-type and encoding upon retrieval
• [x] Accept missing charset, i.e. only text/plain for content-type
• [x] Look into recognition of 301/302 redirects for false text/html detection
• [x] Remove canonical check for now
• [x] Be resilient for invalid utf-8 characters
• [x] Test against some of https://findsecuritycontacts.com/
• [x] Move to a separate subsection
• [ ] Show which domain we retrieved the final security.txt?