release: @formio/core@2.6.6

[johnformio]

Automated sync from formio/nirvana for @formio/core@2.6.6 (monorepo f8874f85de).

After merging, an engineer will tag the release:

git fetch origin
git tag v2.6.6 origin/master
git push origin v2.6.6

Generated by gh-workflows/bin/sync-oss.mjs.

[johnformio]

Closed: sync-oss test plan artifact.