minor #18064 Mentioned the deprecation of deep parameters in UPGRADE files (javiereguiluz)

This PR was squashed before being merged into the 2.8 branch (closes #18064).

Discussion
----------

Mentioned the deprecation of deep parameters in UPGRADE files

| Q             | A
| ------------- | ---
| Branch        | 2.8+
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #18058
| License       | MIT
| Doc PR        | -

Commits
-------

6ea28e6 Mentioned the deprecation of deep parameters in UPGRADE files

UPGRADE-2.8.md

@@ -514,12 +514,12 @@ FrameworkBundle
 
  * The `validator.mapping.cache.apc` service is deprecated, and will be removed in 3.0.
    Use `validator.mapping.cache.doctrine.apc` instead.
-   
- * The ability to pass `apc` as the `framework.validation.cache` configuration key value is deprecated, 
+
+ * The ability to pass `apc` as the `framework.validation.cache` configuration key value is deprecated,
    and will be removed in 3.0. Use `validator.mapping.cache.doctrine.apc` instead:
-   
+
    Before:
-   
+
    ```yaml
    framework:
        validation:
@@ -527,7 +527,7 @@ FrameworkBundle
    ```
 
    After:
-   
+
    ```yaml
    framework:
        validation:
@@ -612,3 +612,21 @@ Yaml
    ```yml
    class: "Foo\\Var"
    ```
+
+HttpFoundation
+--------------
+
+ * Deprecated finding deep items in `ParameterBag::get()`. This may affect you
+   when getting parameters from the `Request` class:
+
+   Before:
+
+   ```php
+   $request->query->get('foo[bar]', null, true);
+   ```
+
+   After:
+
+   ```php
+   $request->query->get('foo')[bar];
+   ```

UPGRADE-3.0.md

@@ -745,7 +745,7 @@ UPGRADE FROM 2.x to 3.0
    The `security.csrf.token_manager` should be used instead.
 
  * The `validator.mapping.cache.apc` service has been removed in favor of the `validator.mapping.cache.doctrine.apc` one.
-   
+
  * The ability to pass `apc` as the `framework.validation.cache` configuration key value has been removed.
    Use `validator.mapping.cache.doctrine.apc` instead:
 
@@ -1022,7 +1022,7 @@ UPGRADE FROM 2.x to 3.0
 
    ```php
    use Symfony\Component\Security\Core\Authorization\Voter\Voter;
-   
+
    class MyVoter extends Voter
    {
        protected function supports($attribute, $object)
@@ -1624,3 +1624,20 @@ UPGRADE FROM 2.x to 3.0
  * `Process::setStdin()` and `Process::getStdin()` have been removed. Use
    `Process::setInput()` and `Process::getInput()` that works the same way.
  * `Process::setInput()` and `ProcessBuilder::setInput()` do not accept non-scalar types.
+
+### HttpFoundation
+
+ * Removed the feature that allowed finding deep items in `ParameterBag::get()`.
+   This may affect you when getting parameters from the `Request` class:
+
+   Before:
+
+   ```php
+   $request->query->get('foo[bar]', null, true);
+   ```
+
+   After:
+
+   ```php
+   $request->query->get('foo')[bar];
+   ```