fix: shards/requirements.txt to reduce vulnerabilities (#2222)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYLINT-568073

shards/requirements.txt

@@ -13,7 +13,7 @@ protobuf==3.9.1
 py==1.8.0
 pyasn1==0.4.7
 pyasn1-modules==0.2.6
-pylint==2.3.1
+pylint==2.5.0
 pymilvus==0.2.10
 #pymilvus-test==0.3.3
 pyparsing==2.4.0