ARROW-13134: [C++][CI] Unpin conda package for aws-sdk-cpp

Closes #11401 from pitrou/ARROW-13134-aws-sdk-unpin

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

ci/conda_env_cpp.txt

@@ -15,8 +15,7 @@
 # specific language governing permissions and limitations
 # under the License.
 
-# workaround for https://issues.apache.org/jira/browse/ARROW-13134
-aws-sdk-cpp<1.9
+aws-sdk-cpp
 benchmark>=1.5.4
 boost-cpp>=1.68.0
 brotli