ci(github-actions/backport): add back "permissions" top section

[bartsmykla]

It seems we have to have contents: read in both, top level and job to make it work

Remediates: https://github.com/kumahq/kuma/security/code-scanning/38

Checklist prior to review

• Link to relevant issue as well as docs and UI issues
  • https://github.com/kumahq/kuma/security/code-scanning/38
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS
  • It won't
• Tests (Unit test, E2E tests, manual test on universal and k8s)
  • No tests
  • Don't forget ci/ labels to run additional/fewer tests
• Do you need to update UPGRADE.md?
  • There is no need
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label)
  • There is no need

Changelog: skip