Update deprecated TokenCache API usage

There was a place in `ManagedIdentityClient` where the `TokenCache.find`
method was still being used, leading to some deprecation warnings being
printed. This updates that to use `TokenCache.search`.

Signed-off-by: Paul Van Eck <paulvaneck@microsoft.com>

Author: pvaneck

msal/managed_identity.py

@@ -298,7 +298,7 @@ def acquire_token_for_client(
         now = time.time()
         if True:  # Attempt cache search even if receiving claims_challenge,
                   # because we want to locate the existing token (if any) and refresh it
-            matches = self._token_cache.find(
+            matches = self._token_cache.search(
                 self._token_cache.CredentialType.ACCESS_TOKEN,
                 target=[resource],
                 query=dict(