GitHub: rename lnd-19-2 branch to tapd-main-branch

The branch was created for a different purpose initially. But because
we're already on lnd 19.2 it is now confusing.
So to make it easier to understand why we need a different litd branch,
we rename it.

Author: guggero

.github/workflows/main.yaml

@@ -21,7 +21,7 @@ env:
 
   GO_VERSION: '1.23.9'
 
-  LITD_ITEST_BRANCH: 'lnd-19-2'
+  LITD_ITEST_BRANCH: 'tapd-main-branch'
 
 jobs:
   #######################