message: updated gcalendar codeowners
diff: /plugins/explore @backstage/reviewers @backstage/sda-se-reviewers
  /plugins/explore-react @backstage/reviewers @backstage/sda-se-reviewers
  /plugins/fossa @backstage/reviewers @backstage/sda-se-reviewers
  +/plugins/gcalendar @backstage/reviewers @szubster @ptychu @kielosz @alexrybch
  /plugins/git-release-manager @backstage/reviewers @erikengervall
  /plugins/home @backstage/reviewers @backstage/techdocs-core
  /plugins/ilert @backstage/reviewers @yacut
  