[JENKINS-66079] Allow setting hookUrl from the global configuration page

[franknarf8]

This PR fixes JENKINS-66079.

It is actually not possible to set hookUrl from the global config page since 2019. The hookUrl setter is not recognized by the bindJSON code since the input parameter type change from URL to String.

The following test crashes with the code prior to my changes :

@Test
public void hookUrlShouldHaveAWriteMethod() throws Throwable {
    PropertyDescriptor hookUrlWrite = PropertyUtils.getPropertyDescriptor(
            new GitHubPluginConfig(new ArrayList<>()), "hookUrl"
    );
    assertNotNull("hookUrl write method should exits", hookUrlWrite.getWriteMethod());
}

Testing done

Re-enabled and modernized the test suite regarding global configs.

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