chore: require amplify cli admin to approve pull requests

[sobolk]

Description of changes

This PR changes CODEOWNERS file to require a member of amplify-cli-admins group to approve the change.

Github evaluates branch protection rules (in a form we have them) in such a way that:

1. It requires quorum (min number of reviewers) this can be anybody with write access to repo.
2. It requires a code owners to approve changes under certain paths. At least one code owner approval fulfills this requirement.

Therefore combination of rules above serves effectively as two tier approval process.

Issue #, if available

Description of how you validated changes

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.