Automatic merge from submit-queue (batch tested with PRs 67061, 66589, 67121, 67149). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Rename KubeConfigFile to Kubeconfig in ClientConnectionConfiguration

**What this PR does / why we need it**:
As discussed with @liggitt we should make the field name and JSON tag consistent, and we concluded `Kubeconfig` and `kubeconfig` is the most consistent naming we have (e.g. wrt `--kubeconfig`), so we're going with that naming for the `ClientConnectionConfiguration` struct. Also, this preserves backwards-compat wrt existing serialized configuration. This fixes the API violation.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
ref: https://github.com/kubernetes/community/pull/2354

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```
/assign  @liggitt @sttts