…eprecate accepting invalid values (GromNaN)

This PR was merged into the 6.3 branch.

Discussion
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[HttpFoundation] Add `ParameterBag::getString()` and deprecate accepting invalid values

| Q             | A
| ------------- | ---
| Branch?       | 6.3
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | yes
| Tickets       | Fix #44787 #48219
| License       | MIT
| Doc PR        | symfony/symfony-docs#...

There were a lot of discussions on #44787 regarding the implementation. The main debate is to determine the right behavior when the value is invalid: try to convert the value, throw an exception, return the default value or the falsy?

I added plenty of test cases for the methods `getAlpha`, `getAlnum`, `getDigits`, `getInt`, ~`getInteger`~, `getString` so that we can discuss the expected result for each input value.

My goals:
- Provide safe methods to get values in the expected type. Example: The parameters being generally of type `string`, the `getString` method is useful to be sure we don't get an array.
- Reduce deprecations on methods that exist since Symfony 2.0, in one of the most popular Symfony component.

How are these getter methods used?
- Most of the time, parameter values are `string` (from routing, query string, request payload). `get` is used but does not validate the return type.
- `getInt` is used for [pagination (UX Autocomplete)](https://github.com/symfony/ux-autocomplete/blob/697f1cb4914480b811d978efe031a6f4a0dc3814/src/Controller/EntityAutocompleteController.php#L41) or [getting an index (EasyAdmin)](https://github.com/EasyCorp/EasyAdminBundle/blob/f210bd1e494b699dec54d2ef29302db1211267b5/src/Context/AdminContext.php#L157-L158)
- I think there was an unidentified breaking change when we introduced return types [#42507](https://github.com/symfony/symfony/pull/42507/files#diff-04f3b8ea71029f48853b804129631aeb9bf3dad7a7a398feb9568bf5397d0e69L117). Methods `getAlpha`, `getAlnum` and `getDigits` could return an array, but their return type have been modified to `string`, resulting in a `TypeError`. [example of use](https://github.com/asaaa1997/Licencjat/blob/8c10abaf87120c904557977ae14284c7d443c9a7/src/Controller/QuizController.php#L164)

Commits
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172f0a7 [HttpFoundation] Add `ParameterBag::getString()` and deprecate accepting invalid values