Allow changes to go.sum *and* go.mod when running dependabot tidy-up (#871)

Transitive dependency changes sometimes do require a change to go.mod
files as well as go.sum. This is already live in the
opentelemetry-go-contrib repo, and this PR brings it here.

Author: evantorrie

.github/workflows/dependabot.yml

@@ -18,7 +18,7 @@ jobs:
       id: modtidy
       with:
         gomods: '**/go.mod'
-        gosum_only: true
+        gomodsum_only: true
     - uses: stefanzweifel/git-auto-commit-action@v4
       id: autocommit
       with: