ci: don't validate dependabot commits

Dependabot doesn't allow configuring the max commit message line length
until dependabot/dependabot-core#2445 is
resolved, so we cannot validate Dependabot commits at this time.

.github/linters/commitlint.config.js

@@ -1,4 +1,7 @@
 module.exports = {
   extends: ['@commitlint/config-conventional'],
-  helpUrl: 'https://www.conventionalcommits.org/'
+  helpUrl: 'https://www.conventionalcommits.org/',
+  // We need this until https://github.com/dependabot/dependabot-core/issues/2445
+  // is resolved.
+  ignores: [(msg) => /Signed-off-by: dependabot\[bot]/m.test(msg)],
 }