Drop use of k8s.io/apimachinery/pkg/util/errors.NewAggregate and replace
to with errors.Join which is available in stdlib since Go 1.20.
This allows to drop one dependency (which itself brings its own
contraints in its go.mod) and use more standard behavior from stdlib.
* feat: Replace default Go user-agent with oauth2-proxy and version
* Add to CHANGELOG
* Make userAgentTransport configurable and composable
* Use correct naming convention for DefaultHTTPClient
* Move version to own package and use named arguments
* Update version path in Makefile
* Fix import path in Makefile
* Change importpath in dist.sh
* Minor style issues
Olivier Mengué
axel7083
Jacob Middag
Blue Falcon
Joel Speed