Match Scala version for 2.13.x in CI

Problem / Solution

We recently upgraded scala to 2.13.6 but neglected to update the version for Github Actions.
This PR fixes that.

Signed-off-by: Ruben Oanta <roanta@twitter.com>

Differential Revision: https://phabricator.twitter.biz/D691500

.github/workflows/continuous-integration.yml

@@ -20,7 +20,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        scala: [2.12.12, 2.13.1]
+        scala: [2.12.12, 2.13.6]
         java: ['1.8', '1.11']
     runs-on: ubuntu-latest
     steps: