Make Spring web configuration classes use proxyBeanMethods=false by default

[bclozel]

We originally discussed this in #22596 but we discarded that because we felt it would make the task harder for people to extend those configuration classes (DelegatingWebFluxConfiguration and
DelegatingWebMvcConfiguration).

After discussing that with @wilkinsona , it seems that it's actually the opposite. If a developer extends one of those with an @Configuration class, that new config class will use proxying, even if the parent class has the proxyBeanMethods=false option. On the other hand, a developer looking at these infrastructure classes can't guess that they can be used with a proxyBeanMethods=false setup, or at least they won't get any guarantee on that while reading the source code.

We should add the proxyBeanMethods=false option on these classes because:

1. This will improve the performance for non-Spring Boot users
2. This will provide hints to developers extending those classes, telling them that it's safe to use without proxying
3. This won't break existing use cases are this is opt-in only

[rstoyanchev]

If a developer extends one of those with an @Configuration class, that new config class will use proxying

Good point. Sounds like there aren't any downsides then. Out of the box is fine, and anything that extends the config has to opt in.