Use deltas report CI action from the dedicated repository

Since the time the CI system for this repository was set up, Arduino's sketch compilation GitHub Actions actions have
been moved to dedicated repositories and the copies left behind in the experimental arduino/actions repository
deprecated.

Author: per1234

.github/workflows/report-size-deltas.yml

@@ -19,4 +19,4 @@ jobs:
 
     steps:
       - name: Comment size deltas reports to PRs
-        uses: arduino/actions/libraries/report-size-deltas@master
+        uses: arduino/report-size-deltas@main