feature #38664 [RateLimiter] Moved classes implementing LimiterInterface to a new namespace (Nyholm)

This PR was squashed before being merged into the 5.2-dev branch.

Discussion
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[RateLimiter] Moved classes implementing LimiterInterface to a new namespace

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no?
| Tickets       |
| License       | MIT
| Doc PR        |

Before we release the RateLimit component.
I think it would be a good idea to put the 7 classes that belongs to a specific strategy in their own "Policy" namespace. It is very likely that it will be more strategies in the future and the `Symfony\Component\RateLimiter` namespace is crowed as it is.

I decided not to put the `CompoundLimiter` in this namespace as it is not a strategy.

Commits
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1e6cea56e4 [RateLimiter] Moved classes implementing LimiterInterface to a new namespace

Author: fabpot

Tests/EventListener/LoginThrottlingListenerTest.php

@@ -37,13 +37,13 @@ protected function setUp(): void
 
         $localLimiter = new RateLimiterFactory([
             'id' => 'login',
-            'strategy' => 'fixed_window',
+            'policy' => 'fixed_window',
             'limit' => 3,
             'interval' => '1 minute',
         ], new InMemoryStorage());
         $globalLimiter = new RateLimiterFactory([
             'id' => 'login',
-            'strategy' => 'fixed_window',
+            'policy' => 'fixed_window',
             'limit' => 6,
             'interval' => '1 minute',
         ], new InMemoryStorage());