chore(deps-dev): upgrade mocha to v11 and fix broken test (#5940)

Upgrades mocha from ^10.x to ^11.x in both .github/actions/import-external-addons
and bin/scan-images to address npm audit findings (brace-expansion, serialize-javascript,
and related transitive vulnerabilities).

Also fixes a pre-existing broken test in bin/scan-images that checked for kotsadm,
which is explicitly excluded via the skipAddons list. Replaced with an ekco version check.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: emosbaugh