Fix conflicting minimum dependencies

There was not possible to install minimum versions
for grpcio-tools==1.51.1 and protobuf==4.21
because these package versions have conflicting dependencies.

The conflict was caused by:
    The user requested protobuf==4.21
    grpcio-tools 1.51.1 depends on protobuf<5.0dev and >=4.21.6

So the patch version of protobuf was updated to mitigate
the conflict.

Signed-off-by: Daniel Zullo <[email protected]>

Author: daniel-zullo-frequenz

pyproject.toml

@@ -32,7 +32,7 @@ dependencies = [
     "grpcio-tools >= 1.51.1, < 2",
     "networkx >= 2.8, < 3",
     "pandas >= 1.5.2, < 2",
-    "protobuf >= 4.21, < 5",
+    "protobuf >= 4.21.6, < 5",
     "pyarrow >= 10.0.1, < 11",
     "pydantic >= 1.9",
     "sympy >= 1.10.1, < 2",