Exlcude root requirements.txt from dependabot

The excluded file is used to set-up the local environment for
Buildbot development.
These requirements must evolve together with the Buildbot version
and the dependencies that we install in the container.

Consequently, this file must be updated manually.

Author: RazvanLiviuVarzaru

.github/dependabot.yml

@@ -13,3 +13,5 @@ updates:
     directory: "/"
     schedule:
       interval: "daily"
+    exclude-paths:
+      - "requirements.txt"