[Validator] Constraint validators now use the 2.5 API

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

See the comments in #11049 for the origin of this PR.

Currently, the 2.5 API needs to use `LegacyExecutionContextFactory` because the constraint validators rely on methods from the old `ExecutionContext` class (like `validate()`, `validateValue()`). Consequently it is impossible to switch to the pure 2.5 API and check whether all calls to deprecated methods were removed from application code. This is fixed now.

This PR also introduces a complete test suite to test each constraint validator against all three APIs: 2.4, 2.5-BC and 2.5. Currently, some tests are not executed yet when running the complete test suite is run. I expect this to be fixed soon (ticket: sebastianbergmann/phpunit#529, pr: sebastianbergmann/phpunit#1327).

No, because you don't always want to validate in the same context. For example, imagine validating some constraints and, depending on the result, adding one single combined violation. Then you need to validate in a different context.

This was in fact one of the motivations for the API change.