doc: call out http(s).globalAgent defaults

Despite the http.Agent stating:

The default http.globalAgent that is used by http.request() has all of these values set to their respective defaults.

this isn't true anymore since node.js 19. Both, the http and the https globalAgent now set { keepAlive: true, scheduling: 'lifo', timeout: 5000 } as options. 'lifo' is the default anyway, but keepAlive is turned off and no timeout is set on new Agent().

Document the diverging behavior in the globalAgent sections, remove the false statement from http.Agent section, and extend the changelog to call out the timeout change as well.

Closes #48821 Closes #51115