Replace hand-authored social icons with verified open-source SVGs

github/orcid/mastodon/bluesky/matrix: official Simple Icons paths
(CC0), fetched from simple-icons/simple-icons. linkedin: Simple Icons
no longer includes it (removed from the project entirely, likely a
brand/legal request), so sourced from Tabler Icons' brand-linkedin
(filled variant, MIT license) instead for a closer visual match to
the other filled-style icons than their outline variant.

Previous icons were reconstructed from memory as an approximation,
not verified against any actual licensed source.

templates/icons/linkedin.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" role="img" aria-hidden="true" focusable="false"><path fill="currentColor" d="M20.447 20.452h-3.554v-5.569c0-1.328-.027-3.037-1.852-3.037-1.853 0-2.136 1.445-2.136 2.939v5.667H9.351V9h3.414v1.561h.046c.477-.9 1.637-1.85 3.37-1.85 3.601 0 4.267 2.37 4.267 5.455v6.286zM5.337 7.433c-1.144 0-2.063-.926-2.063-2.065 0-1.138.92-2.063 2.063-2.063 1.14 0 2.064.925 2.064 2.063 0 1.139-.925 2.065-2.064 2.065zm1.782 13.019H3.555V9h3.564v11.452zM22.225 0H1.771C.792 0 0 .774 0 1.729v20.542C0 23.227.792 24 1.771 24h20.451C23.2 24 24 23.227 24 22.271V1.729C24 .774 23.2 0 22.222 0h.003z"/></svg>
+<svg role="img" viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg" fill="currentColor" aria-hidden="true" focusable="false"><title>LinkedIn</title><path d="M17 2a5 5 0 0 1 5 5v10a5 5 0 0 1 -5 5h-10a5 5 0 0 1 -5 -5v-10a5 5 0 0 1 5 -5zm-9 8a1 1 0 0 0 -1 1v5a1 1 0 0 0 2 0v-5a1 1 0 0 0 -1 -1m6 0a3 3 0 0 0 -1.168 .236l-.125 .057a1 1 0 0 0 -1.707 .707v5a1 1 0 0 0 2 0v-3a1 1 0 0 1 2 0v3a1 1 0 0 0 2 0v-3a3 3 0 0 0 -3 -3m-6 -3a1 1 0 0 0 -.993 .883l-.007 .127a1 1 0 0 0 1.993 .117l.007 -.127a1 1 0 0 0 -1 -1"/></svg>