Charts: update kubectl to 1.26.7

What does this MR do and why?

Describe in detail what your merge request does and why.

Updates kubectl binary in the Charts build image from
1.20.15 to 1.26.7.

This matches the version currently specified in the
[charts .tool-versions
file](https://gitlab.com/gitlab-org/charts/gitlab/-/blob/e52340272537c7a9c0a677b80d0ba6ca5bfe8933/.tool-versions#L1).

Notably, this new version gives us the ability to use the `-l` flag
with `kubectl rollout status` to get the rollout status of multiple
workloads matching a set of labels.

Test plan

• Confirm new version is installed in the image: https://gitlab.com/gitlab-org/gitlab-build-images/-/jobs/5090797687#L125
• Confirm base image works in a Charts CI pipeline: gitlab-org/charts/gitlab!3393 (closed)

Checklist

• (If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.