csl-libs: bump github workflows scala version

Problem / Solution

We recently bumped our scala version to 2.13.6 but neglected to update it
for our github actions CI integration. This patch addresses this for the
rest of our libs.

Differential Revision: https://phabricator.twitter.biz/D691506

.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: