👷 Update CI for cpp20 branch

Problem:
- CI directives refer to the `main` branch.

Solution:
- Update CI to run on the `cpp20` branch.

Author: elbeno

.github/workflows/unit_tests.yml

@@ -5,9 +5,9 @@ on:
   workflow_dispatch:
   merge_group:
   push:
-    branches: [ main ]
+    branches: [ cpp20 ]
   pull_request:
-    branches: [ main ]
+    branches: [ cpp20 ]
 
 env:
   DEBIAN_FRONTEND: noninteractive

.github/workflows/usage_test.yml

@@ -5,9 +5,9 @@ on:
   workflow_dispatch:
   merge_group:
   push:
-    branches: [ main ]
+    branches: [ cpp20 ]
   pull_request:
-    branches: [ main ]
+    branches: [ cpp20 ]
 
 env:
   DEBIAN_FRONTEND: noninteractive