Deprecate using doctrine/cache in favour of PSR-6

UPGRADE-2.2.md

@@ -0,0 +1,8 @@
+UPGRADE FROM 2.1 to 2.2
+=======================
+
+* Deprecated using doctrine/cache for metadata caching. The `setCacheDriver` and
+  `getCacheDriver` methods in `Doctrine\Persistence\Mapping\AbstractMetadata`
+  have been deprecated. Please use `getCache` and `setCache` with a PSR-6 
+  implementation instead. Note that even after switching to PSR-6,
+  `getCacheDriver` will return a cache instance that wraps the PSR-6 cache.

composer.json

@@ -24,7 +24,9 @@
         "doctrine/annotations": "^1.0",
         "doctrine/cache": "^1.0",
         "doctrine/collections": "^1.0",
-        "doctrine/event-manager": "^1.0"
+        "doctrine/event-manager": "^1.0",
+        "psr/cache": "^1.0",
+        "symfony/cache": "^4.4|^5.0"
     },
     "require-dev": {
         "composer/package-versions-deprecated": "^1.11",