If you use a non http based update center (e.g. file:// or other) then the UpdateCenter validation breaks.

```
2022-12-09 09:31:50.119+0000 [id=107] WARNING o.e.j.s.h.ContextHandler$Context#log: Error while serving http://localhost:8080/manage/pluginManager/checkUpdateSiteUrl
java.lang.ClassCastException: class sun.net.www.protocol.file.FileURLConnection cannot be cast to class java.net.HttpURLConnection (sun.net.www.protocol.file.FileURLConnection and java.net.HttpURLConnection are in module java.base of loader 'bootstrap')
at hudson.PluginManager.doCheckUpdateSiteUrl(PluginManager.java:1968)
at java.base/java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:710)
at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:397)
Caused: java.lang.reflect.InvocationTargetException
```

The UpdateCenter check should not assume all UC are HTTP based.

Steps to reproduce

1. download a valid update-center.json file and store it locally on the file system (/tmp/foo.json)
2. start Jenkins
3. goto the plugin manager
4. in the Update Site URL enter a URL such as file:/tmp/foo.json

Expected results

Validation passes

Actual results

Stack trasce and angry Jenkins

Caused by https://github.com/jenkinsci/jenkins/pull/6886