Upgrade CoreDNS to 1.2.6

[prameshj]

this contains a fix in the loop plugin that was incorrectly detecting a loop when upstream dns was unavailable.
coredns/coredns#2255

[k8s-ci-robot]

Hi @prameshj. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[prameshj]

/assign @justinsb
/assign @bowei
/assign @MrHohn

[justinsb]

/ok-to-test

/lgtm

I scanned over the vendor changes and they look reasonable also (and include the fix we really want)

[johnbelamaric]

/lgtm

[bowei]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bowei, johnbelamaric, justinsb, prameshj

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [bowei]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment