This commit ensures that a primary JSR 303 and Spring Validator will be
exposed if the auto-configuration kicks in. As `LocalValidatorFactoryBean`
exposes 3 contracts (JSR-303 `Validator` and `ValidatorFactory` as well as
the `Spring` validator one), this makes sure that those types can be
injected by type.

`LocalValidatorFactoryBean` exposes 3 contracts and we're only checking
for the absence of a `javax.validation.Validator` to auto-configure a
`LocalValidatorFactoryBean`. If no standard JSR validator exists but a
Spring's `Validator` exists and is primary, we shouldn't flag the
auto-configured one as `@Primary`. Previous iterations on this feature
have made sure that we'll auto-configure at most one
`javax.validation.Validator` so not flagging it `@Primary` is no problem.

This commit also restores and adds tests that validates
`ValidationAutoConfiguration` will configure a JSR validator even if a
Spring Validator is present.

This effectively fixes gh-8495 in a different way.

Closes gh-8979
Closes gh-8976