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`.
```
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