Adapt to HttpComponentClientHttpRequestFactory changes

[bclozel]

As of spring-projects/spring-framework#28925, Spring Framework doesn't configure the Apache HttpComponents client 4.x, as this generation is not maintained anymore. The implementation has switched to using the httpclient5 generation.

Besides adapting our auto-configuration and various tests, we also need to consider:

• how we should adapt our RestTemplateBuilder in light of the deprecation of the setReadTimeout method on the request factory
• whether we want to completely remove our usage of this client (in devtools, buildpacks and CLI support)
• whether dependency management for the 4.x generation is still required as a result

The 5.1.x documentation has interesting bits of documentation about migrating to the "classic" API as well as typical client usage examples.

This change should be reflected in our own migration guide.

[wilkinsona]

I've labeled this as an enhancement as I think we'll want it to be listed in the changelog.