Automatic merge from submit-queue (batch tested with PRs 47286, 47729)

Set default User-Agent on http probe

**Set a default User-Agent on `httpGet` probes**:

Currently the default Go HTTP client sets a `User-Agent` specific to the language and version, but every Go client has the same one.  In Kubernetes, users can override the User-Agent by setting a header in their probe definition, but its tedious to do this everywhere.

This PR changes the default User-Agent only if not set (or removed) in the probe definition.

**Which issue this PR fixes** 
fixes #29025

**Special notes for your reviewer**:

**Release note**:

```release-note
Change default `httpGet` probe `User-Agent` to `kube-probe/<version major.minor>` if none specified, overriding the default Go `User-Agent`.
```