While trying to use '.' !== $key[0] instead of str_starts_with($key, '.') in #47680, I noticed some tests were failing due to the usage of numeric parameter names in the fixtures.

This leads to inconsistent behavior since the following code: $parameterBag->set(10, 10), will first cast the name 10 to string because of the method signature, but will then cast back to integer when using it as an array key in https://github.com/symfony/symfony/blob/6.2/src/Symfony/Component/DependencyInjection/ParameterBag/ParameterBag.php#L89.
Because Symfony does not use strict types, it can be really tricky.

This PR propose to deprecate using such names to be able to properly throw in 7.0.