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

@@ -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

@@ -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];
+   ```